For the first time in a while (despite having good results of late), I decided to look into my opponents in detail. I noticed he passed ALL the time - so rather than sticking to my usual plan, I changed a few Def Plays - my DBs looked better in general than his WRs, so I chose lots of man-marking systems (as has previously been suggested). They scored 44pts :(

I noticed their best players were CBs (which mark WRs). So to counter this I decided to play to my strengths (running plays) which I've noticed thanks to recent stat analysis etc. My rush yardage/completion dropped from usual ~7yds to ~4yds! My passing which I assumed would be now poor increased a bit to ~6yds! We only scored 30pts :(

I know there's lots of ifs/buts and extra factors which could cause these things and without someone analysing the game in great depth, we may never know whether they simply were a better team (we had about similar record in league so far) or whether I screwed up...but is there anything obvious I got wrong in my strategy?

Sometimes they're just going to put up passing yards regardless of what you do. Also I'm sure giving up 227 punt return yards was part of the reason they scored 44 points. You can't really base anything off of one single sim. Especially if you are using plays you haven't used before, because your players will not be familiar with those plays.

Thanks
No I understand about one off game.
Yes they got loads of yards there. How do I stop the big yardage gained in punt returns? My special return team right? I think quite a few were injured etc but assumed they would simply be replaced. Maybe I should have focused on them as I've never even touched those special team line up.
Live and learn I guess

Basically you need to set up your depth chart in a certain way. You want fast guys that can tackle well as your gunners for the most part.

Thanks
Like I say I neglected that part of team and focused on offense and defense but I guess without gunners (?) Etc I will never win

Some times changing up your PB before a game will backfire because your team won't be familiar with the plays.

Thanks but to be fair I didn't change it too much. Set a little bit more running cos of his corner backs and chose a few man def plays
Looking fwd to making amends in next match - really enjoying this game :)

Setting the ST depth can be tricky at times, especially if you are only carrying the minimum (or close) amount of players on your team.
The question then becomes whether you want to place your starters on STs and risk them getting injured, or playing backups that aren't as good.\
A good idea is to look at your opponents' previous games to see how their returner did and adjust accordingly.
It is also a good idea to follow the health/performance of your players and adjust the depth as necessary.