Slap them in da face j/k.
Players form changes, some players perform great at 5stars and then drop down once they ge 6/7 stars
imho you should continue trainng them and try to fix their skill points in a way they gain their form back.
I had players that used to get ratings of 6-7 in 1st season, 8-9 in 2nd season so ya ;P Their skills are immportant, don't give it up.