Do you have the right BIOS in the BIOS folder? (I used SCPH1001.bin and it worked fine for me) you can also add .iso to the es_systems.cfg file if your iso files dont show up (or do a full binary update and they should show up- but make a back up first if you do that)
https://github.com/retropie/RetroPie-Setup/wiki/Playstation-1
