Fill-Valve Float intermittent failure
I have had these same valves get to the point where they quit working reliably so yours may be on it's way. However I would venture to guess that one of the following could contribute to intermittent failure.


The float on your particular valve may have been deformed slightly in its manufacturing process to that it is just a little out of ideal shape. This lead to the thing binding on the riser post if it was in just the right position.
There may be some molding seam flash on either the float or the post that leads to a binding problem with the float when it rises with the water fill.
Check to see if there is any possibility of mineral deposit build up in the working slide area of the float. This could lead to sticking or binding.
Make sure that the float is fully away from the sides of the tank so that it never has the opportunity to scrape on the tank side.
Check the upper valve lever arm hinge under the upper cover (turn water off before removing) and make sure it has nothing that could cause it to want to stay in the down position making to force the float to stay down.


RECOMMENDED COURSE OF ACTION

These valves are really pretty inexpensive and it only takes about 30 minutes to replace one [provided your shut off valve is functional :^) ] so I recommend that you just get a new one and swap out the intermittent one.