Simple problem. First, download the XWE beta with the autocapitalization option. Then, go to View (on top) > Options > Entry List > Check the box that says "don't autocapitalize lump names", and you're set. Now, go into your wad, and see if you have two Ax and Bx entries (PLAYA1, PLAYB1, etc.). Change the ones with the tailspickup frames to lowercase "a" and lowercase "b". It should totally fix your issue.