Replaced Fill Valve and toilet won't stay full

Hi,
I replaced the entire fill valve on my toilet. When I flush, everything looks fine. Fills to the water line on the tank/overflow tube and stops. The next time I try to use the toilet, the tank is empty and I have to flush the toilet so that the tank fills so that I can flush it again.
I can't figure out where the water is going. There is no water on the floor under the toilet. The water level in the bowl is fine. I can't hear anything leaking that would suggest the flapper needs to be replaced. The tank never fills up either.

I cannot figure this one out. Any ideas?

Is the float rubbing on anything? that could hold it up faking a full tank. Does the fill valve always run after you flush? As noted, unless the valve is sticking (normally the float), if the tank leaks through the flapper, the fill valve would get activated automatically. The float could be rubbing on the flush rod, and when you press the lever again, it releases the float. This shouldn't be too hard to figure out.

From what you've described, you have 2 problems:

1. A leaky flapper

2. A stuck fill valve

Keep the tank cover off so you can watch exactly what is going on. Once you get more info, let us know so we can better diagnose the problem.

Good thought about the stuck fill valve. I will leave the lid off of the tank and watch it tonight.
I will let you know what I see tonight......