Used stamps exchanges.

I have sent many stamp swaps in a number of swap groups and only seem to be having issues here. I had one rating low and they photographed the back of the stamps. No stamps had folds rips or tears. some may have had a slight discolouration on back where they were stuck to an envelope. Can anyone please add insight so I can make sure I am following rules. I am very particular about the stamps I send

Thank you so much for your comment. It does help me for future swaps. It is not my intention to send stamps that are not up to standard and am just a bit disappointed my swap partner did not elaborate on the issues. It does make it hard to make sure you do not repeat the error. I thought it was a bit harsh but we live and learn. Happy Swapping and again thank you for the reply.

I have enlarged that particular image which is rather small.
Based on that, I couldn't find any folds or tears either.

I think the case might have been that 6 stamps have their corners missing.
As for the others, I suppose they might have had either multiple short perfs, thins, or both.
It's difficult to say for sure with this image quality.

Discoloration from envelopes, while not dangerous like foxing/mould, is also frowned upon by some people. But the stamps which backs are shown don't seem to show significant discoloration.