I've had instances where selling or buying one player changed the team dynamics and overall form struggled. identifying your key contributors and maintaining decent rotation will help but I usually keep new players out of the first team for a couple days until they have had a ferw practice matches under their belt with their new teammates and have acclimatized. I also don't play players that I've bought post group stage in CL playoffs as their form never seems to go above 7 having missed the early stages of the competition. You may have encountered teams that are paying higher or max win bonuses than you which can also cause a drop in overall team form. Be careful when instituting win bonuses as I had have problems with motivation when adjusting it too dramatically in the past.