it could be the primer/moisturizer you’re using before applying. im not very good with skin types and making recommendations off of it, but i’d consider finding a primer that’s good for the type of skin you have (oily, dry, combination). i have combination skin so for me i use a elf primer, foundation on my face but not really under my eyes, then a light amount of concealer under eyes (a little goes a long way) blended. i try to avoid the foundation in places i’m concealing bc it’s less product which means less caking/separation. then take a sponge and bake it with some translucent powder. sometimes the separation is inevitable and part of the process but this is what helps me!