Heat & Glo SL-550TRS-IPI-D Burner Quits After 10 mins

First Timer Here so be gentle.
I am in the trades and took on a customers used Heat & Glo fireplace thinking that after installing quite a few gas fireplaces that I could get there older unit up and running.
So after building a new mantle and new tile work i went to work on tuning up the fireplace.
The home owners had me convert the unit from LP to Nat gas. I went to my local supplier and purchased the natural gas conversion kit and installed it.
So at the time it was hooked up to a wall thermostat and i turned the unit on. The pilot sparked and lit the burner which at the time did not seem to respond to the flame control. The flame was also very low, (max 3"). So i called my supplier and they recommended a new valve.
So I bought a new valve and a new remote unit to do away with the wall mount thermostat as per home owners request.
I installed all of that including a new fan and everything seemed to work fine except for 2 things.
First the flame control barely upped the flame, and secound, the flame was still very low.
The home owners were fine with that so i went off to another job, only to get a call back about a month later.
So apparently after a little over a month the "Thermo Mode" is not working properly by not igniting when the temp drops below the setting , but what is worse is that the unit after igniting in manual, goes off by itself after running for about ten minutes.
I've read almost all the posts here, as well as the troubleshooting section of the manual.
Only thing I can think of is low gas supply, and how that can happen after a month of regular use is mind boggling.
Anyone have a situation like this please advise me on the fix, and thank you in advance.

OK as we all suspected it was a supply issue, and to the question of the run, appliances, size, etc. that comes into play as well.
Firstly the fireplace is the furthest from the main line, approximately 35 linear feet of 1/2 pipe connected to a 3/4" main trunk line.
There is a water heater first in line, then stove, then upstairs fireplace, and finally the downstairs fireplace where we are talking about.
Everything is branched off the 3/4" line.
So getting back to the site I took off the supply line valve in the appliance and low and behold there was a big chunk of pipe thread compound partially clogging the opening. I suspect it came loose over time and made its way to the valve.
Went to check the pressure on the supply line with my gauge only to find the glass had broken and was rendered in operable.
At that point i couldn't leave the home owners with the gas off to get a new gauge so i just went ahead and put everything back together.
Upon start up i was hoping for a larger flame, but it was still pretty low lending to the theory the the pressure/volume is still too low.
However the good news is that the fireplace is seeming to run fine and isn't shutting itself off.
Looks like the only fix for the low flame would be to run some new 3/4" supply closer to the fireplace, so wish me luck pitching that to the home owners.
Thanks again for your Help.