Husband's Mom avoids me?

I was recently remarried to a wonderful man. His Mother does some puzzling things, however. She is notoriously shy, as is my husband. My husband repeatedly says that I make no effort to have a relationship with his mom. But, I call her, leave messages (sometimes), etc. She claims she leaves her phone on silent/in the car, etc or is busy and doesn't see my calls. But, she never calls back. My husband says she does the same to him and doesn't answer, but 9/10 he can get her to answer. When we're in person she talks to me (sometimes more times than others) but is usually quiet. I think she dislikes me or is repulsed by me, when my husband says she is busy and doesn't see my calls. I think they're excuses that my husband is siding with her on and I resent him telling me that I make no efforts. I've asked her to hang out, or inquired about such activities that would make it known that I want to be cordial and be a good daughter in law (she has never referred to me as that, only "his wife" when my husband is in our presence). She just says that sounds fun, but never follows up or does anything that shows she cares about getting to know me. Am I right in thinking she is purposely doing this to spite me and that she dislikes me or is my husband right, she's busy and forgetful where her phone is concerned? I told my husband I'm done pursuing anything and he is blaming me.