Repeating Cell protection

I have a problem with a small block of cells on a spreadsheet. The cells are unlocked in a sheet that is protected and are intended for pasting data copied from an email each day.
The problem is that every time data is pasted the cells protect themselves so that it isn't possible to paste data the following day.
I thought I was going potty when this first started happening and repeatedly reset the format for theses cells to "Unlocked", but every time the result was the same; they locked themselves when data was pasted.
What is going on here and how can I rectify it?