Check to see if your circulating pump is spinning, Grundfos and other common types used here have a cap in the centre of the pump body, remove it with a screwdriver and see if you can see the impellor spinning.
If the main burner is coming on to heat your domestic water, the thermostat (when turned on) puts power to the circ. pump, and pushes the water thru the panels, so check the t/stat and pump.

Nearly 20 years Plumbing and appliance maintenance and hydronic units still baffle me on occasion, we don't see many of them...........................................maybe I could organise a service call? lol