Brian,
I’ve got a tiny (girl-sized) biscuit joiner and I also hold it down with my hand. Another thing that can cause a problem: if the joiner isn’t powered up fully before plunging, it will sometimes lift or move. (I sometimes have a problem with impatience. ) Keep practicing!
L/W
“Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.” Benjamin Franklin