What is the reason a fluorescent shop light can't be flush mounted? I'm thinking air flow to cool the ballast?

I want to install several more shop lights in my garage, but to clear the overhead door they need to be mounted directly parallel to the joist. All the lights I looked at say not to flush mount, suspend only. I have two lights in my basement flush mounted, but they are perpendicular to the joists, so I guess they get air flow as I haven't had any problems. All other lights are suspended. Any thoughts, ideas, etc... Thanks

Clarification---the lights are mounted ON the joists

The reason is air flow, to cool the ballast that is inside each fluorescent fixture. Always was mandatory for the older magnetic ballasts, they would and could get piping hot . Many fixtures offer a spacer as accessory. Even one inch is fine.

TK,
Thanks for the info. I was thinking of going the spacer route if air flow was the issue, which you confirmed. These are the older mag ballasts, they get warm but not hot-but better safe than sorry...

When magnetic ballasts fail, believe me, they get HOT. I have no more fingerprints.