Slowing down rpm on training programme

Hi. I am doing the FTP 6week training programme. I am finding that every time I slow down (after a period of higher rpm) the W ends up jumping around as I gradually slow to the given rpm. I do not go under the required rpm which is usually 85rpm but as the W jumps around whilst I slow I do not then achieve the star for that section of the training - it even drops to zero at times suggesting I have stopped.
Any idea why this is?