Using unibios4? Correctly named? Other bios files correctly named? There are 2 or 3 variations of filenames out there on the internet's, worth checking that the bios names are correct. You can find the correct names on the git page for the neogeo core. I had this issue, even though the bios files were fine on other emulators.
In addition, several bios files must be placed in the NEOGEO folder for the core to function properly:
000-lo.lo
sfix.sfix
sp-s2.sp1 (MVS)
neo-epo.sp1 (AES)
uni-bios.rom
Sometimes these files may be named slightly differently depending on where they are obtained, but they must be renamed to match the filenames above to work with MiSTer. You may choose between using original system BIOS (sp-s2.sp1/neo-epo.sp1) and uni-bios.rom. Using uni-bios is recommended, and can be obtained here.
Also, try a few versions back of the neogeo core. Ensure you're cold booting between ROM selections on large ROMs.
Make sure you've only got a single directory/location for ROMs on the sd - people have had issues with multiple locations and ROM files.