IME, a common problem...
that velox tape is not the stickiest stuff in the world. reapplying it a time or two doesn't help matters either. and a tire that is difficult to mount can cause it to shift. anyway, i've used Johnson&Johnson (any drugstore) adhesive tape (two wraps) in the past, and it has held up well. cleaning the rim before applying any rim tape doesn't hurt either, IME. good luck.