i have this too - seems that picodrive doesnt like the .md files inside of 90% of my zips where as DrMd plays them fine.
just to be a little more helpful, this can be fixed by unzipping your roms and using ms-dos, batch renaming the file extensions using:
ren *.md *.bin
then rezipping to their individual archives.
probably quite obvious, but might help someone...now if only picodrive had a prettier GUI, it would be perfect