As title states, the flames get smaller after a few minutes. I have an insert I got from Lowes. Starts fine but after 8-10 minutes the flames reduce by a whole lot.

Edit: also flame cuts off completely after about 15 minutes. Just leaves the pilot on.

In-the day DIY Home Depot propane logs experiences similar
(never again, now pampered by AAA Overhead Door & Fireplace A+ kits, install and service seldom used)

A. RESET
[ol]

Find the pilot light assembly and its reset switch. The furnace's pilot light is usually positioned at the bottom of the unit by a knob. ...

Set the reset switch to off. Turn the knob to the "Off" setting. ...

Wait a moment. ...

Relight the pilot light. ...

failed?

[/ol]B. PRESSURE LOW
1. Brush & clean gas outlets, pilot, thermo then vacuum (every year for us - ugh)
2. Check supply pressure, e.g. low on fuel or very cold propane tank else nat.gas service lines if possible
C. THERMO
1. Our #1 problem! Once cleaned, repositioning it will either get it into proper temperature zones
or reconnect a loose wore/connection
2. Order/replace thermo ... although this solution lasted only 1-2 years, even throwing money at them
D. ARRAGEMENT
1. Some log kits are VERY sensitive to exact placement for either venting or thermos. Study install diagram to conformance, even if not aesthetic (few are)

Took it apart to look for this stuff. Tried to run it without the logs. It had two pilot lights and two thermocouples. Only one pilot lights up. I cleaned out the tube but still won't light. Could that be my issue?

My main issure was always the thermo - so EZ to agree with some of that research -
thermo, safety gadgets installed & propane refill tank issues drove me away from DIY
to AAA Fire & propane-delivered Suburban gas.

So sorry, had the same issues in that Feb 15F weather UGH

I was on my way to load up on Duraflame logs, then gather fallen wood from back acre, then disconnect propane gadgets from hell when AAA showed up and saved the day.