The Everdrive will support up to 256 items per folder. If you have more than this they will not appear in the file select list on the drive. Make sure you have less than 256 total items in the rom folder.
A couple of question to help narrow down what could be going on if that's not the case.
First. Where did you buy the Everdrive N8? Was it bought through krikzz or retrogate? Or did you buy elsewhere. It's possible you have a clone everdrive and that could cause issues.
Second. Have you tried using a different SD Card? The Everdrive n8 can be temperamental about certain brands of sd card.
Third. Have you tried loading the ROMs from a different source (i.e. GoodNES or no-intro)? It could be a bad source for the ROMs.
Please take all these things in to account, test, and report back.