I have this problem from time to time. I disconnect to take a shower. The try and put the cover on and it will not go on. I try to reconnect my pump and it will not go on either. The only solution is to insert a new line. Does anyone else have this problem?
It’s so interesting to know someone else has this problem!!.
It really weirded me out at first and I was pretty pissed,but then,I noticed that the top part of the infusion set,without anything connected to it,can move around,so you just need to screw it a bit and voila 
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I just had this problem today,right after showering.my advice is,to avoid unnecessary site changing due to the plaster coming off when screwing the top part back,dry off the site completely,use your fingers for support to keep the lower part from coming off and use your other hand to screw the top part,don’t be afraid to put some power into the move,it won’t move easily.
hope it helps,and tell us how it goes
Never had his happen. But I do squeeze the tabs on the cover while turning and this helps, I presume.