I agree with the others:
- get their teeth checked (properly, with an otoscope to see the back ones)
- try a brand new bag of the same pellets - the open bag may have something wrong / different about it.
As long as they are eating everything else and other hard treats, poo is OK, etc - I would be less worried. Get them weighed and teeth checked just in case, and monitor all input and output. Hay eating is good and the veg is OK, so just keep going. They don't actually need the pellets, but suddenly not eating them indicates that something isn't right somewhere - good on you for spotting it and acting on it quickly.