A year and a half ago we applied Kilz original on a sub floor in a house we are renovating. Then recently we went back and painted latex porch paint over the Kilz. The Porch paint comes off is something is slid across the floor. We thought the porch paint would be very durable. Why is the porch paint coming up?
1-1/2 years is a long time for primer, it has hardened too much and your paint won't stick. Plus as mention by Dave it could be a problem with your paint. Water based paint won't stick well to old hard oil base primer.
After removing the loose paint you will need to do some sanding and repriming before applying your paint.