Up until recently, things had going fine, and then it stopped filling the tank. So I jiggled it, or turned the faucet or shower or both on and it would work. But only for a little while, then the little tube would stop filling and dribble. Now it's not even dribbling. I pulled it out and ran water through it, so it's not clogged. The thing it was attached to wasn't putting out any water, not even a drop. I'm pretty sure it says "Fluidmaster 400 Series Anti-Syphon". I've already gone to their site and no luck. What can I do?

I think the valve inside the filler is blocked. I just bought a new valve, and flapper in a kit at home depot for about $ 12. I would replace the whole thing. That's what I did anyway.

It is clogged up with dirt inside the fill valve. The Fluidmaster A400 fill valve is easy to service: First, close water supply to the toilet. Then, separate fill valve in the middle of the adjustment column. It is held together by a small plastic ring. You may need both hands to pull it up and to free the upper portion of the valve free. Than look inside the fill valve. There will be debris. Clean it, reinstall the upper portion of fill valve to its original height. Don't forget snap in the ring that secures upper and lower portion of the fill valve together . Open water supply. Your toilet tank should refill.

Sounds like you have a defective seal in the fill valve.

To replace seal
Turn off water at stop valve below toilet tank.
Remove tank top and place some where that it won't get broken
Reach under float, lift and grasp shaft.
Float must be in upmost position
Turn cap 1/4 turn counter clock wise.
Lift off cap and replace seal.

If you want to replace entire fill valve

Slide height adjustment ring up.
Pull fill valve off lower shaft.
Slide new valve on to shaft to proper height .
Slide adjustment ring down to lock shaft in place.