Old Eljer Low Tank One-Piece Toilet - Water Not Filling Bowl

I am trying to help my mom. Her house was built in 1980 and she has an Eljer Watersaver Low Tank One Piece Closet (toilet). I have the original installation manual.

The problem is that the bowl does not fill completely after a flush. It fills about 1/3 full. A second flush fills it another 1/3, and a third flush fills it the rest of the way up.

What could cause this and how do I fix it?

Thanks very much!
Carol

flush

If it takes 3 flushes to fill it, then you must be getting very short flushes, and refilling the bowl is the task of the fill valve's "refill" tube which directs water to the bowl while the tank is filling.

Thanks for the start! So how does one fix short flushes and/or the refill tube? What exactly do I need to do to the toilet to make it behave?!