I have a Logitech DFP, and I sometimes have to open up the pedal unit, de-fluff it, and start again. I sometimes get no reaction when I press either pedal, as well as them not going all the way.
There are either 9 or 11 screws in the pedal unit, the first time you un-do them they are really really well screwed in so take care.
When separating the top and bottom halves, be careful, as the pedals and their motors etc just rest in position, there are no screws etc to hold them there when the unit is in two halves.
There could be a lot of grease in there too, so be careful you don't get it everywhere.
The first time I did it took me half an hour, but every subsequent time it is only 5 minutes.