Sometimes you can have a different zone for the underfloor heating which will mean a separate motorised valve.If this valve is this valve is jammed in the closed position it will not open up to allow circulation round the underfloor pipes.Another thing is underfloor heating pipes tend to get blocked easily so it might be worth while checking the state of the system water.Check if the pump is running ok & if its on a low setting turn it up to the maximun settingand see if it makes any difference.Also check if you havnt got two pumps on the system as some underfloor heating circuits have their own pump which will only kick in when the underfloor heating stat has been turned on.If a thermostat/timer for the underfloor heating is installed check it to make sure it activates the boiler.I could go on all day so try these and goodluck