natural gas valve

My sister is having a problem with her gas water heater. The pilot light was not staying lit so she had someone replace the thermocouple. This did not fix th problem so she had someone else come in and replace the valve. The replacement valve did not work at all. The old valve was rated at 5 psi and the new one was rated for 3 psi. Would the psi rating matter on a natural gas water heater. If not what else could the problem be.

Is it not staying lit when you try to light the pilot, or does it go out after you turn it on once?

How much trial and error are you going to before you call a professional? Keep screwing around with something you know nothing about can kill you especially if it's gas!