Why does my custom rig float if R15 animations move the LowerTorso?

I am trying to use R15 animations with my custom R15 rig, and they work well, until the animation moves the LowerTorso. This causes my custom rig to float above the ground sometimes and makes the animation look unnatural. I just want my rig to stay grounded. I have tried adjusting the HipHeight, but this doesn’t lead to much success. Can someone please provide any insight on how to fix this or why it may be happening?