It is well known that in this game, the player form reflects a player's success rate and contribution in a match.
I consider 6 being subpar, 7 is normal, 8 is excellent. Getting a 9 is on fire.

There is a hidden factor for each player, I call it "motivation", which is the greatest deciding factor about player performance. Its impact is so great that it overshadows quality and morale. Meaning, a low quality and low morale player with high motivation can easily beat a player with high quality high morale but low motivation.

Low motivation is the main reason some player enter "slumps" and fail to get good form continously in matchs.

The obvious thing is that a player with low motivation (constantly getting 6 or below) have a very high chance of missing penalty kick (I say over 80%), regardless of quality or SA. All other actions too: failing to tackle, making fouls for no reason, making mistake passing to an opponent, etc.

There is no definative way to change the motivation of a player. Training is one thing that helps. I also discovered recently that subs out a player with bad motivation during half time, if he had done bad on that match, appears to improve that player's motivation later. Player with low motivation would often reverse upon transfer: a player with bad form directly from market would usually perform excellent instantly once he enter your team, and vice versa.

It is hard to identify the motivation of players because its value is hidden, but there are signs for it. The obvious one is that a player lose morale faster when the motivation is low. A player with high motivation may instead have his morale increasing over time, often there are cases players with zero morale gaining back to 100% on their own after playing some matchs because their motivation is near the peak.

In my experience, it is harder to get a good form if your team goes against a stronger team, as the chance of succeeding any action becoming lower and the chance to make mistakes becomes higher.

A player with high motivation could still fail and get a low form if he is out-played in a match, but the case will be isolated and inconsistence.
A player with low motivation would constantly getting bad results no matter how weak his opponents are.

Keep a player with good performance on the pitch while keeping under-performing players at the bench... This is supposed to be the most basic thing a good manager should do but often overlooked.