http://waterheatertimer.org/Water-heater-recirculation-system.htmlThe circulation system should draw hot water directly from the water heater as shown on illustrations on link above.
The Grundfos pump works on a thermostat, and some work on a timer.
Can you hear the pump working?
There should be a humming sound.
Check the timer -or- thermostat setting.
It is possible the lines are clogged with sediment if you have hard water.
If I was going to fix the problem:
1) check thermostat or timer
2) listen for pump sound
3) check electricity going to unit
4) take apart the pipes and inspect for clogging
If there is a clog, I would flush the line or soak the part in CLR (calcium lime rust remover)
The circulation pumps are sealed and there is no way to take apart the motor, timer, or thermostat for repairs.
add a comment if you need more assistance.