i have had issues like this which seem to stem from LACK of humidity, so not sure what your case is. make sure your geckos have a water dish just in case of dehydration. you can put a bit of lukewarm water and a paper towel in a deli cup and have the gecko sit in there a couple hours, then take a damp q-tip to it and gently try to rub any skin off. you could also use a shedding aid spray to help moisturize it. more often than not, it will lose the tip, and sometimes it will heal with a bit of tail missing, other times it just gets worse and the gecko needs to drop the tail.
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4.1 snakes, 4.6.1 crested geckos, 0.1 gargoyle gecko, 0.0.2 red-eyed tree frogs, 2.0 devon rex cats, 1.0 betta, 0.1 sun conure, 1.0 lovebird