Why wont oven reach temperature?

Bad thermostat? Bad gas valve so oven never gets enough gas to reach temperturs? And can you put the dial on wrong so it's not set at zero when off. It could be that the thermostat is faulty, or it could be down to a simple human error such as having accidentally left the oven door ajar. Another possibility might be that your gas pressure is low- if the latter, this would be a fault in the pipeline and not with the oven itself. Whatever the reason, I don't advise you to try and fix it yourself, for safety's sake- call out a service engineer.