Is it possible for, say, a person to have reached the third jhana after a strong A&P/second jhana meditation period, then due to the lack of DN concentration be hard pressed to reach any jhanas at all and yet remain a DN yogi for some period of time? Maybe that's part of the process of becoming equanimous?

Or, rather, must a DN yogi be more or less reaching the third jhana each meditation until finally they find themself chipping away at Reob during meditation to break into the fourth jhana? Is it that linear?