Certified GKH Gardening Expert
Sometimes, this can be chalked up to genetics. If you suspect this is the case, then it will just take time, and proper care.
If they are in different areas, then the microclimate could be different in each location.
As long as they receive proper care, they will produce blooms as they should. This article will help you to give them that care: https://www.gardeningknowhow.com/houseplants/african-violet/african-violet-care.htm