moopy
There could be a multitude of reasons for this. You should ask a qualified gas professional to check the following.
Is there adequate working pressure at the gas appliance?
Is the gas valve working correctly?
Is the pilot light the correct length, color and playing on the thermocouple?
As the problem is intermittent he should ensure the gas pipe sizing to every appliance in the property is correct.
Also it would be useful if you could post close up pictures of the pilot assembly.
