Radiators work some days and not others...

We have just moved to new house which has two Vaillant boilers (no indication as to model nos, no instruction manuals, no help from previous owner). One works most of the radiators in the house and is fine, the other works the hot water (no problem there either) and also (we think) the bathroom radiators. The bathroom radiators were working for the first few weeks, then stopped for a few days, then started again for no apparent reason, then stopped, then came on one morning but have now stopped again and have not come on for four days. I have vented them, they had some air in them, but are still completely cold. The boiler is run off an electronic timer, which seems to operate correctly, the heating symbol and the hot water symbol both come on on the boiler. Any ideas?

dont know if its relevant but saw this earlier thought it might help/confuse further

kevplumb said:

in days of old bg (before gas )
you cant control a solid fuel boiler as quickly
so one rad (or towel rail ) was always on with the water
ie heat leak it lost enough heat to stop the boiler boiling
well that was the theory
does the towel rail get hot when only on hot water ?

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