But reserve players don't expect to play week in-week out.
Not suggesting these other suggestions are all wrong but imo this is assuming that there is quite a complex mechanism behind the form bar when in fact it seems almost random.
In my current game I got fed up with crappy form so employed the best trainer for form available, there was a slight upturn but still what form my team decides to be in doesnt have any real bearing on anything.
All my team are at least average in terms of happiness, and yet for three weeks running they all had (literally every man even the 17 yr old youths who barely play) yellow or red arrows, then we have to play Man Utd (who are second) and hey presto I lose one of my best defenders to a training injury and not one player is on yellow with 3 first teamers on purples and the rest on greens and blues.
There is literally no reason for that stunning drop in form from 1 week to the next, I won the prior game comfortably, used subs to rest tired players did nothing different to the previous 3 weeks, yet the game decides my form will drop.
Id love to say I believe it goes by in game results and player happiness but it doesnt seem to have any bearing at all, it seems almost completely random and having a good form stat and decent trainer just give you a better random chance, thats what ive experienced anyway.