Filters probably. There are a couple things you can check. 1) clones - if you turn off clones, you will lose a bunch of roms (although that's not necessarily a bad thing) 2) verification - if you are using a different version of MAME and roms, some might not be compatible and be removed from the list 3) mahjong, mature, japan, etc - there are other filters that will remove games you probably don't want to play/display in your list All filters can be tweaked or turned off. Please be aware that after changing a filter setting, you must update your list for any changes to take affect.