To add to @panze’s note, it’s worth checking if the 60 job has ‘Enforce Sequential Rendering’ enabled (details in the docs), as that will keep that job from getting interrupted.
Thinking of that, is the 60 job Interruptible? If it’s not, it won’t get bumped out of the way by higher priority jobs. (Details on that here).
That last thing to check would be if the machine you’d expect to pick up the 90 job is even allowed to grab it, but since suspending the 60 seems to work, we don’t need to worry about that. (If we were worried, a great first step would be using the Find Render Candidates tool to see what’s what.)