The main reason for a toilet tank not filling or filling slowly is due to the toilet fill valve. The most common reasons for the fill valve to slow down or no longer fill the tank after the flush are debris issues and the length of time a fill valve has been in use.

Why won’t my toilet fill back up with water?

When a toilet is not filling up with water it is a sign of a faulty fill valve, wrong height of the toilet float, a leaking flapper, cracked overflow tube or low water pressure.”

How do I make the toilet bowl fill up with more water?

Quote from the video:
Quote from Youtube video: So all that you have to do to raise the water level in your toilet bowl is to use a screwdriver. Like this to turn this to raise it up now you'll notice if you turn it in one direction.

How do you adjust toilet fill valve height?

Quote from the video:
Quote from Youtube video: Place your thumb on the side of the arm and twist the cap and the arm together. And then it's going to lift right off the top of the valve remove the refill hose.