Could be retained shed from last time (will make tail fall off it not removed). It can be hard to see and you have to look at the actual snake not the shed skin very hard (maybe with a magnifying glass) to see if all the shed came off the tip of the tail. After the snake sheds this time you should check it out and if you see skin you should put a little bit of warm water in a bowl (that a lid can be put on) with a wash cloth and put the snake in, put the lid on and leave the snake for 10 mins. Should loosen the skin up and come off. But if it doesn't it should be loose enough for you to gently pull it off yourself.
If it isn't a stuck shed it may be from where the tail tip has already fallen off. Or is an old injury. My youngest snake has this problem and I have to help her shed the tail tip every time because they smashed her tail at the pet shop and the tail tip died and fell off. It feels hard to the touch, but doesn't seem to bother her. It looks better with each shed but still feels hard.