Atari 7800 ROMs won't open even when BizHawk does

[ThePowerPlayer]

Hi there, yesterday I was downloading media for the Atari 7800, and when I downloaded a Dig Dug ROM and tried to open it in HyperSpin, the emulator opened but not the ROM. I can't understand what went wrong. I gave HyperHQ the directory to the BizHawk emulator and my ROM folder, and did the same with RocketLauncher. I checked that Bizhawk supported the .a78 extension in both applications. For some reason, this has only happened for the Atari 7800 - on every other system I use BizHawk with, it works fine, and when I opened the ROM manually, it worked perfectly. Is there anything I'm doing wrong that can help solve this problem? Help would be greatly appreciated.