Washing the plastic with soap might help. GS should stick to most surfaces though unless it is particularly oily, even 2-year old GS will.
Cutting holes or scratching the surface you want to stick the GS to helps too, as the putty can be pushed into the depressions and get a better purchase there. I sometimes use GS/BS to pin models together and find that holes/scratches help out.
In the case of a CD dread beard, I shave off any existing beard and leave the surface rough. I place a sausage of GS, rolled somewhat evenly, but with one tapered end. The tapered end goes on the face end of the dwarf and then using a knife dab little cuts along one side of the sausage, then alternately on the other side, a la Xander’s BFSP and other conversions. I think I finish up the beard a bit differently. I cut very slight lines across the beard (side to side) to add more “hairy” detail. When I’m done, I use a dentist’s scaling pick to press a hole at the bottom of the beard cylinder, so it has a tubular end.
Or course, not many of my CD beards have been seen, so you might not want to listen to what I’ve said…