If it was left to cure in fairly cold conditions, this might be the cause of your issue. Epoxy can become very brittle and less likely to bond well in such conditions
If you can, leave it to cure somewhere indoors, if you've been working in a garage or similar.
After the initial coating has cured, it might also be worth applying two or three additional coats after each one has reached a firm tacky stage, to save on having to sand each time. The overall bond between the different coats should be much better too