Games working fine, 30 mins later they're "not valid roms"

[Legend10269]

Hi, I bought Drastic a few days ago, today I copied onto my phone's (Sony Xperia Z3) SD Card a load of .nds and .zip files, I copied them over one by one, as I'd read doing them in a group can corrupt them. I had a few problems with some of the files (getting stuck on the loading screen, or not showing up at all) but I put that down to bad roms more than anything, the majority played perfectly though. Happy that I'd set everything up I closed Drastic and started doing some other things on my phone (installed epsxe and copied some roms over for that). After I did that I tried to play one of my DS games on Drastic and every single game is now saying it's not a valid rom. I haven't changed anything to do with Drastic or the roms since it was working 30 mins ago, so I've got no idea what could be causing the problem. Please help, thanks.