Do butt plugs typically fall out during PIV sex? If they're not supposed to, are we doing something wrong?

Wife (39) and I (42M) started exploring anal within the last few years. We started slow with just rubbing during PIV and built up to where we are now-ENJOYING anal sex. About a year ago while going down on her I began pressing and rubbing around her ass when she grabbed my head and had one of the most impressive orgasms I've ever seen. My idea was to buy a vibrating plug for her ass that would free up my hands for other places. During oral it's fantastic-if she wants to wear it and ride me, the damn thing pops out. Sometimes it happens right away, other times it's mid-ride which causes it to rest on my balls which is either FANTASTIC or painful.

We're happy to try it while spooning or doing doggy. She LOVES the way it feels but gets super frustrated when it slides or pops out. Thanks for any tips or advice!!