are they chewing out of the corners?
if so, i had the same problem. what was happening was the rats were chewing any corners that were raised slightly(dented in) they were able to get ahold of it with their teeth and once they made a small hole the easier it was the chew all the way through......
So what i had to do was get flawless tubs (look at all of them at lowe's and make sure there is no dents at all) and add more bedding, so if there were any dents in there, its out of sight out of mind!

and if they weren't chewing out of the corners, i don't know what to tell you, LOL

hope that helps ; )

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