No gas coming from hob?

I have a Cannon Salisbury and only 2 of the hobs are working (the two front ones). I think it is the gas valves controled by the knobs as there is not enough gas coming out when you switch it on and you can't light it with a match but obviously the other 2 are fine. There is something coming out when you switch them on but not enough For a while one of them was working but the flame was very small, even on full but then it packed up.
Any ideas on how to repair it?