Take backs on bridesmaids?

I have two friends who are part of my bridal party. We’ve been friends for years, and it’s been rocky, but we’ve always been there for each other. I’m even suppose to go on a trip for one of their birthdays. But I recently had a party that they didn’t go to or even showed interest in going, and I just asked if they wanted to do something for my birthday and I didn’t receive a response. It makes me feel like they really don’t want to hang out with me, and if that’s the case, why are they even my bridesmaids? I need people who are going to be there for me on my big day. I just want to know if I should keep them in the wedding, and if not, how do I tell them?