A couple of things to consider Ruby..................you know the boiler will have a output control...............probably two.........one for the heating, and one for the hot water. It's usual to have the heating control on about three quarters, but in your case it might have to be turned up to full. Also, look in the manual for the location of the circulating pump............these are usually switchable to different speeds. This is really scraping the bottom of the barrel, but, if the pump is set on too low a speed, then "possibly" the heated water is not circulating efficiently. I'm really trying to make the best use of an underpowered system.
Good luck