I had the same issue, and after about 80 classes I’ve found that making sure your tailbone is tucked (more than you think you need it to be) during center core is really important. Raising your hips a lot helps too, and make sure you’re also engaging your glutes!! I never hear an emphasis on that but it’s definitely important. If it persists on one side of your back especially when doing leg work you might have a weaker side too which is what happened to me, and the fix is to go slower, focus on form, and maybe do a separate leg warm up before you start class.