vokera gas valve will not open?

I have a mynute 14e that I am running on propane. It was would run but would lock out after a while. I put a monometer on the inlet test nipple and I am getting 37 mbar when I try to fire it up it drops to 35.5 / 36 mbar then locks out. I had feeling that the gas valve was not opening so I turned off the gas at the cock on the boiler. I was still getting 37 mbar. I then turned the burner pressure up to max and tried to fire it the monometer drops to 0 as it the valve has opened then when it fails to fire it shoots back up to 36 / 37 mbar. Can anyone help me please?

Thanks
FRAN.