Toilet Tank doesn't reliably fill?

OK, all together now. The thing at the left, inside the tank, is the Fluidmaster "fill valve". The thing in the middle inside the tank is the "flush valve", commonly with a "flapper". Either can hang up and not move properly, but I'll guess you are referring to the first one; they aren't made as well as they used to be. Both need a little tweaking when installed. Identify which is the problem.

Since this is something you paid for recently, then at least call your plumber, and ask what that might cost. A good one will just take care of it, as he will want more of your business. He will have new valves of both types with him, as they are very common parts. If that doesn't fly, you can purchase both in a "kit" for about $12, with decent DIY instructions. Before I retired from hardware, I could also do a hands-on walk-through with you to do those "tweaks" I mentioned, or a DIY refit, for that matter. Good to learn about this stuff.

I think your tank fill valve is likely faulty or clogged with something. It's fairly easy to replace.

The flapper valve is hanging in the "up" position?

There's probably a "burr" of plastic or rubber where it snaps onto the valve seat, or maybe even on the valve seat "ears". Turn off the water, remove the flapper, and inspect it. Inspect the "ears", too; feel around with your fingers.

When you re-install it, there should be complete freedom of movement, up and down.

get the installer to stand behind his work and products = = = it sounds to me that the float is binding and if it is possible to use "" emory cloth sand paper "" to free up that bind then that is the the correct sand paper [ works in water or when in wet conditions ] === do not use any vaseline or lubricant and the water in the tank will cause that to gum up and then definately not work ..... hope this helps ....

Spray it with WD40 mixed with 1 oz of lighter fluid.

is the little cup rubbing on something like the side of tank or is it free on all sides (should be free) and is it sat in the little cradle coorrectly and square or is it tilted i.e only clipped correctly on one side

call the people who put it in and tell them to fix it