Well, I and other people on this board can go through and make comments about individual games; alternatively, we can give suggestions for more general things.
General comments:
Plateaus like that are normal. You go up, you reach a peak ranking, then you fall back down and feel like you're stuck. It always happens. Things which you should do:
1) Don't worry! This is normal! It's not a reflection on you or anything.
2) Don't get discouraged! If your ranking stresses you out, play unranked games, or make an alt, or something. Or play IRL instead of online. Or however you like - make sure the game stays fun!
3) There are a few ways to keep improving. One is to just keep playing more games. Two is to record your losses and try to get analysis of them. Three is to try out strategies yourself and experiment - if you want to know how well a strategy works, pull up a solitaire game and try it out, or learn to use the sims to check out strategies, or something. Any or all of them are good.
Specific game comments:
Game 1. You went Oracle/Secret Chamber/Harvest/Laboratory/Nobles/Tunnel. Your opponent went for a much purer Lab/SC/Tunnel, with nobles at $6. The SC were his only discarder.
Two comments. I *think* - though I am not totally sure - that you need fewer tunnels in a tunnel game. The goal is to use tunnels to bootstrap to gold, not to flood your deck with them for points, though the points are nice too. You thought you won the 5 tunnels vs 3 split, but really that's not an advantage. I think. Someone correct me if I'm wrong, or mDuo you can try it out in sims or solitaire.
The results were unspectacular. The game ended on turn 24, which is a *long* game - a standard BM+X with a good enabler should get to 6 or 7 provinces far before then, so BM+Oracle might have been better. It's not as clear because of the nobles and tunnels for VP, perhaps the VP from those would have been enough to overcome a 6-2 and definitely a 5-3 province split, so I'm not 100% convinced. Close game.
One thing is that the other guy was more focused - he went for Lab/SC/Tunnels pretty hard. You were a little more scattered, got an oracle and only one lab and a harvest. I suspect that's not optimal - you should either focus on lab/sc/tunnel like he did, or skip that and go oracle/bm, but I suspect that a mixed bag of tunnel enablers would be less effective than a focus on the best one. (Or, in this case, worse than oracle-BM in the first place...)
Hmm. Total count: you had 5 terminals, 0 villages, 2 nobles in your deck, he had 2 terminals and 5 nobles.
Game 2: Goons game.
I do see one clear mistake early - you opened Walled Village. I could understand this if you didn't want a Militia at all in your deck, but you picked one up on the first shuffle anyway, and getting the Militia first and the WV later is better than the other way around! On the other hand, later in the game, there was a point (after turn 10) where your deck had 1 village and 6 (!!) terminals!
I'm confused about your turn 13. Didn't you have a scheme in hand? Why didn't you play it?
Oh, and at the end: you have 5 villages, 12 terminals, and 1 nobles; opponent has 5 villages, 10 terminals, and 4 nobles.
I think the courtyards were a mistake. You didn't have room for extra terminals, they clashed as it is. You needed the terminal space for wharves. I think one thing from this game which MIGHT be generalizable to more is that you need to keep track of how many villages and terminals you have or want. 6 terminals on 1 village is a recipe for collisions.
Game 3: well, you ignored familiars, but didn't counter them. Your opponent was also the first one to get a Salvager for trashing. And since it was a Colony game, you couldn't try to use something like Wharf to race to a lead before the curses/slow trashing kicked in. Oh, and I note that you spent all your early turns buying either Wharves or Crossroads - you didn't get a single coin-producing card until turn 8 (salvager) and the gold on turn 9.
This one was easier to analyze - it looked like you saw one powerful thing going on (a wharf engine with crossroads) and neglected the rest of the board. You needed to either get familiars or counter them - either match the potion/crossroads open of your opponent, or open wharf/crossroads and get a potion on the next shuffle (possibly better), or at the very least try to get salvagers though I suspect that wouldn't have worked well. Since this board was a long game, the universities would be a fine consolation after the familiar race was over, and salvagers could salvage the potion, so I think that getting the potion would have been the clearly right play.
You said that you tried to play defense with salvagers, but you didn't get your first salvager until turn 8, so it's a pretty late defense.
Game 4: another easy one to analyze, because it was a simple case of wrong strategy. Chapel is NOT a good setup for a big money game; chapel->Money+drawer is slower than just going the money strategy in the first place. And, Masquerade is a nice counter to chapel, because you never want to be left with a hand of only expensive things. This was a definite masq+money game; potentially a late-game remodel would help or potentially a mid-game council room as a second terminal card draw would help, but I'm pretty sure chapel is a trap here.
Game 5: ouch, another really close one. Here you lost via islands, despite winning the province split. I would have guessed that doublejack or jack+some islands on $4 for tiebreaking would have been faster than remaking expands and expanding royal seals, but if you drew a lot of $5 and $7, well, do whatever you can. I do think that jack+silver would have been a better opening.