Heating Problem

Lately when I turn on the Gas boiler it no longer heats the water in the water cylinder like it used to . This cylinder is located in the "Hot press" and have checked that any valves may have been inadvertently closed off . Now to have a shower I have to heat
the cylinder electrically by using the internal elements in the cylinder. Now water comes through nice and hot when I do this so the hot water is getting to the shower and the mixer on my shower works fine. now I have the gas on all day and the water doesn't heat up to anything like it did before. has anyone had a problem like this before.

System could be air locked ,Pump not working or stat not working would be better to get a plumber in to have a look.(you should not have to wait long now a days)