Item permission levels revoked

HI all, 

I have a sharepoint list using Sharepoint 2016 with about 23 items, each item is assigned to 1 person in my organisation. What I mean is that I have given permission to that 1 person to edit just that 1 item. Usually works fine, but lately just randomly, 1 or 2 people wont be able to access it. They get an error messaging saying they do not have permission. So I go and check the item and sure enough they are listed as having edit permission. So I remove them and re-add them and bingo, it suddently works for them. I dont understand why randomly it just occurs and removing and re-adding resolves the issue. Its very frustrating and quite counter productive. 

BTW I cannot give them all access to the whole list because they arent allowed to see eachother's items. 

Anyone experience similar? Any ideas?

Thanks

Luke