New unit (and house) 18 months ago, I flushed the hot water heater, can't get the recirculation pump working again.
I know they can get air bubbles and fail to create suction and pump through - I've tried various versions of turning on and off supply with the recirc pump both on and off, and tried opening and closing that red flush valve quickly to break vacuum - one time I thought I could hear a swirl of water like it caught, but still no quick hot water at the kitchen sink 40' away.
The pump is definitely on- I can feel it vibrate and it's warm but not hot - same as it was before. I just have to wait 90 seconds for hot water at the kitchen sink.
And of course, I asked the plumbers NOT to do the pump/instant hot water thing at all during the build. But they didn't read their own documents and installed a 3/4" circuit rather than a 1/2" direct line. And the kitchen sink was last in line. Which I didn't realize until the drywall was up. So basically had to do the pump at that point, which the plumbers treated as a change order since it wasn't originally spec'd. I was salty.
But how do you troubleshoot?