It's mapped right that's the only game I usually play that just stopped starting for some reason. I installed a new mame of same vesion in new folder and tried new roms and working Roms off other machine and now those say missing rom or chd files and don't even load now. Not sur what else to try except deleting all and starting new my other machine works fine with same version of Roms and mame 152
IIRC Galaga uses the Midway sound system roms which were moved to their own zip awhile back so it might be you are missing them (ie. Namco50.zip,Namco51.zip,52,53, 54, namco62.zip) Get those and see if it helps !
EDIT : I tried the midway Galaga with those removed and it shows it missing namco51 and namco54 so that could be the problem.