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OK, so it's a bit of a first world problem, but here goes...
We've got a combi boiler that feeds two heating loops - one for underfloor heating in the kitchen/diner and one for the rads in the rest of the house. Each circuit has a motorised valve to control where the flow goes, and there's a bypass with a pressure valve on it as well as one built into the boiler.
The problem is that in the mornings we're increasingly finding that the rads are stone cold, but then if we drop the main thermostat below room temp then reset it the rads come on without fail. Rads have not yet shown this problem in the evenings & the underfloor has been fine.
My gut feeling is a problem with the motorised valve on the c/h loop, maybe interference from the underfloor side (u/floor comes on before the rads). Any other ideas or things to check before I call the plumber.
Motorised valves often go tits-up. It's easy to test them by getting someone to change the thermostat settings while watching the valve to see if it reacts. You should hear a zzzzizzz noise and see the little lever moving. The usual problem is the motor; they aren't cheap but they are easy to replace as long as you have space for access. Less usual is that the actual wet part of the valve is sticking, which means a drain-down for replacement. You will have to decide whether or not to go for a motor or the full monty and the plumber's merchant will probably warn you that they won't refund the cost of the motor if that turns out not to be the problem.
BTW if it's a problem you can lock that little lever manually in the open position, there's a notch in the slot in the casing.