Had the same sticking float problem on one of my bikes for years. Just kept a spanner handy when starting to give the carb a tap to release the float. Eventually, after a few inspections where I had found nothing untoward, I realised the float was sticking on the rib for the cover retaining screw - as in the pic.
Basically the problem was the 'flash' where the two halves of the plastic float are joined together. Quick touch-up with a stanley knife effected a permanent cure. Strange thing was, it only happened after the bike had been standing unused for a while. Clearly in those instances, remaining petrol had evaporated (I run my carbs nearly dry before stopping the engine) and the float was then able to get itself stuck at the bottom.
