securing foot on the hip for river guard

Hey all,
I've been doing the ronin thing since March and things are finally starting to really click with my half guard. Now that I'm more flexible I'm starting to work with the rubber guard but I'm having a consistent problem with securing my foot on the hip any and all advice would be appreciated.

What happens when you try to secure your foot on the hip?

I can't wedge my heal into his hip or he tenses up and I slide of the side of his hip