Gas hob

Do you have an instantaneous gas water heater or combination boiler?

If so it would suggest the high gas demand from the water heating is depriving the the hob of adequate gas flow.

There are a number of possible causes of this, but beware what you have a potentially dangerous situation - especially of the hob does not have flame failure sensors and could continue to supply gas to unlit burners.

If this is a new fault and previously it was ok, check your main gas tap is fully on. If it is then you likely have a fault with your main governor, or the local supply. Contact your supplier.

If this is not new, then the above may still apply, or you gas pipes in the house are not correctly sized for the needs of the installation.

To verify if this is the case, see:

Yep, inadequate gas pipe size. Have you recently had a boiler replaced by some cowboy outfit?