8 man dungeons shouldn't be eligible for duty finder. They probably leave because you don't have a PLD or a BLM/SMN, both which are almost essential to killing ifrit.

Actually, I don't think any level50 dungeon should be. They're all extremely challenging and require competent players, teamwork, and gear. No ILVL requirement on queueing for garuda/titan, can get someone in full af + level 20 accessories who is basically useless. There's really no reason for duty finder to be usable for them, when you need certain classes for 8 mans and certain ilvl of gear to attempt the harder content.

Leveling up dungeons should be the only dungeons you can duty finder, because they're not as challenging, are required for story, and it's harder to find people for lower level content the longer the game matures.