Why does sunscreen always ball up when I apply it over my moisturizer?

Beauty Tips For Body Care
I've tried out two physical sunscreens so far and both times they've balled up over my moisturizer (Aveeno Eczema Therapy) and I end up having tiny white clumps all over my face. Why does that happen? Is it a certain ingredient that causes it or am I just not waiting long enough for the moisturizer to completely dry?
Girls Blog 2015
Submitted by kai-funk