I have this same issue on my G34. Turns out my right thumb is either riding the slide release or interfering just enough to keep it from engaging the slide. I removed the extended slide release and it helped but still happens occasionally. It doesn't bother me enough to try and change my grip.
I like it! I am constantly tripping over and/or moving my tumbler around the garage floor. Never thought about putting together a dedicated tumbling station. Smart!
Some of my best results came from being loose and having fun. That did not mean I forgot entirely about the plan or the task at hand, just that I was more relaxed and less focused on the outcome.
Welcome Rick!
Join us out at Cavern Cove in Woodville, AL with the Rocket City Practical Shooters. You can shoot or watch anything from USPSA, IDPA, Speed Steel, 3Gun, etc. Pick your poison. Great group of people and a ton of fun.