How do I diagnose a hot water recirculation problem?
I agree, your symptoms seems to indicate the problem is the recirculation system, not the heaters.

First, locate the recirculating pump.   It could be near the water heaters, or at the farthest fixture from the water heaters.  Your most distant bathroom may be a good place to look first.   Look under the vanity or other enclosed places for a smallish device (size of a closed fist or bigger) with two water lines and an electric line.

It may have a timer that is mis-set, it may be unplugged, or plugged into a dead outlet (perhaps a tripped GFCI).

Other possibilities would be a valve closed either near the recirculator or at the water heaters (if the recirculator has a dedicated return the whole way back).

If none of these lead you to the problem, you may have a defective pump, or a more subtle problem... it may be time to call a plumber.