I was left with a number of African Violet plants when my house-mate moved away. I have not changed their environment at all. In fall I re-potted them, started some babies, and broke apart one large and old plant. One of the offshoots from the old plant is now blooming but three of the others developed tight bunches of hard, curled, deformed leaves at the center of the plant. Oh...I also tried feeding them with an African Violet fertilizer that I have no idea of it's age. I used the correct dilution and frequency but it may be past it's date. Could the fertilizer have caused these deformed leaves? I've stopped using it and am trying to be patient as I wait for the rest of the plants to recover and resume blooming. I just cut some of the deformed leaves out of the middle of one of the well-established plants in hope that new growth will come.
Any other suggestions?

There may be many reasons. Irregular watering, potted soil, poor drainage, pot size, mite ...
All 4 plants grow in the same soil?
Is it possible to see the image?

I have one doing that so I'm about to change potting soil after 6 months. It is a nice green but with those stunted center leaves. It gets the same watering, fertilizer & light as my others. I've isolated it in another room just in case it's a bug.

Usually, the tight curly center is a sign of too much light.Too much light inhibits AV growth, makes the leaves small and deformed. Even though you didn't change anything, it takes time to develop that center.

Cold stress is one cause of hard, curled, and also tightly bunched center leaves on They prefer a warm location with consistent temperatures of around 70ºF both day and night; bright, indirect light and they should always be kept away from the cold drafts of windows and doors. Always allow the water to come to room temperature before watering.

Leaf curl is also a sign of mite infestation. The mites aren't noticeable but you'd likely see stunted growth and damage in the center of the plant, as well as damaged blooms from mites feeding on them.