Check the reach of the screw
reach, is the distance between the head and where the thread starts.
In order to pull the board down tight the reach of the screw has to turn freely in the top board will pulling into the framing below. If the threads are still in contact with the top board and you try to screw the board down tight, something has to give. 1 screw may strip out the hole in the board
2 screw may strip out hole in framing below
3 screw breaks
If nothing else can be done drilling the top board with drill sized so threads of screw down grab the top board.