I have this problem sometimes. The front of my hair is 3a/3b but the back often looks 3a or even 2c! I used to think that it was a problem with my product application, but now I think it's my layers back there- maybe they're too long? While I wait to get it cut I tend to be more liberal and thorough with products back there and that has helped. I also skip curl every once in a while if I don't think it's curling properly.
What products are you using?
Burt's Bees Super Shiny low poo & condish
Fave products: KCCC, KCKT, CK, Re:Coil, Joico, Aloe, BRHG