YOU USE THE WRONG TOWEL -- all towels aren't equal. Get one that's thick enough to offer a very different challenge than normal Chins, but grippy enough to give you a chance. Nothing too new and smooth.
YOU DON'T CHALK UP -- and slide right off! It's woirth the effort to buy some chalk to get the extra muscle stimulation from doing it well.
YOU GO FOR TOO MUCH -- Lower your ambitions when you start out on these, get used to the griping challenge and don't judge yourself if your reps are waaay down compared to your normal Chin Ups, it's normal.
YOU RACE IT -- and you lose range of motion and control as a result. You need to keep these slow to get the most benefit from them.
YOU DON'T DO IT -- you know who you are! You avoid these as you can't do it and you feel like you're going backwards. The trick is to appreciate just how much harder towel-grip chins are, then to humbly work on the easier progressions.