Robertshaw fireplace gas control valve - pilot can't stay on

My gas fireplace has a robert shaw gas control valve similar to a Robertshaw 710-502 Dual Gas Valve Millivolt Low Profile. The pilot wouldn't stay on once I stopped pushing the knob down. I measured the mV coming from the thermopile and got around 400 mV. Then I bought a White Rodgers 600 - 750 mV 36" thermopile (G01A-132) and replaced it. It can go as high as 500 mV. But same issue. Pilot wouldn't stay on. I attached an old 1.5 volt battery in series with the thermopile and got 1. 4 V reading (without the pilot on). Then I on the pilot and it remained lit without even waiting for 10 seconds (once knob is not depressed). It works. The fireplace now works. However it isnt safe and I wouldnt use it this way. Is the issue with the thermopile being around 500 mV? Should I get something that should generate 700 - 750 mV? Second question, does it matter whether the red line of the thermopile gets connected to TH/TP and the white goes to TP or vice versa. Thanks for any help.

source https://hvac-talk.com/vbb/threads/2235609-Robertshaw-fireplace-gas-control-valve-pilot-can-t-stay-on?goto=newpost