Gas hob not staying lit?

I have a problem on a Stoves gas hob. In total it has 8 gas burners. The manual recommends litting the gas and holding it on full for about 15 seconds. This should then make sure the gas supply stays on but when I let the control go the gas cuts off again. I think it is a problem with the thermocoupler but don't know how to replace these. The hob was brand new and never worked correctly. Nearly all the burners have the same problem. I am limited to about 1 or which work consistently. It is extremely frustrating esp when cooking larger dinners like christmas. From what I know there is a safety shut off for the gas so that if the flame blows out the gas will be cut off to prevent a dangerous situation. I feel that these sensors which detect the flame heat are not functioning correctly. Not sure if I can get replacements or not... any advice would be greatly appreciated.