Actually the problem is usually getting it to stick to the tape to lift it at all, so you're one step ahead of the game. Sign Warehouse comments that to get 631 to stick real good you may have to heat the transfer tape after applying and re-squeegee it.
Maybe your Mactac isn't adhesive enough to stick to the substrate. Try a lower tack tape, but then you may run into the opposite problem. Actually, you should try applying it to glass, first. If you can get it to stick to glass but not the wall, your final substrate may be the problem. You may need to clean the wall and if it's a textured/matte finish you may not be able to do it at all, at least not easily.
Maybe your Mactac isn't adhesive enough to stick to the substrate. Try a lower tack tape, but then you may run into the opposite problem. Actually, you should try applying it to glass, first. If you can get it to stick to glass but not the wall, your final substrate may be the problem. You may need to clean the wall and if it's a textured/matte finish you may not be able to do it at all, at least not easily.