Don't know how I missed this but I did. I have this exact same smoker, and have several thoughts and solutions on this. I have experienced these one time or another over the years using propane smokers. You may have already tried some of these.
1. The stock thermometer is JUNK. Mine was off by over 50 degrees, and was very erratic.
Solution, Replace it with a good thermometer Or add a digital probe placed threw a potato on a grate. I use a digital probe with remote display.
2. Wind and cold will effect the temps of this unit BIG TIME.
Solution, Construct a wind break around unit. I use 4 sections of insulated duct work around mine.
3. Valve on LPG tank not open fully. If not opened fully it will restrict the flow of gas.
Solution, Open valve on LPG tank 100%
4. Bad flame. Check flame with door closed. You can see flame threw hole on left side of burner assembly. A good flame will be; blue, have little yellow tips, and be between 1- 2 inches high.
Solution, Clean burner assembly including burner holes I use a wire brush, and pipe cleaners. Clean venturi tube, I use a small bottle brush. Clean screens on venturi tube, I use old tooth brush
5. Smoke stack damper not open. You need the damper open to allow flow threw smoker.
Solution, Open smoke stack damper
6. Heavily loaded smoker. I have noticed that when I have the smoker heavily loaded my temps run lower.
Solution, smoke 1/2 as much and do 2 smokes, add some lit charcoal to the chip pan, use a dry water pan or 1 filled with sand.
7. Smoker not preheated. I get better temps when I run my smoker on high for 15- 30 min with no water pan or smoke tray. I then add the water pan fill with boiling water and smoke tray and run till I get TBS, then add my meat.
Solution, Preheat smoker
8. Regulator safety activated. If you open the valve on smoker, then valve on the LPG tank, the safety on regulator can activate, and restrict your flow of propane. It can also happen if you open the LPG valve too fast. Quick way to tell if activated is watch your flame as you turn the valve on smoker. If flame hardly moves or not at all its probably activated. Will also normally be a small flame.
Solution, Close valve on smoker and LPG tank. Remove the hose from LPG tank. Wait approximately 30 seconds, reattach hose. Slowly open LPG tank valve to full open. Press the electronic ignition as you open valve on smoker.
9. Side vents on smoker open too much. I find as 53rdcard does I get better temps with the vents closed.
Solution, Close the vents
10. Bad propane/LPG tank. Believe it or not you can get bad propane, and or LPG tank. Your BTU's will suffer due to this. I have had this happen several times when I was using BlueRino bottle exchange. Since I have switched to a local suppler, that fills my bottles, I have had no problems.
Solution, switch tanks
11. LPG tank low on gas. When my LPG tank gets low on gas I see a drop in temp. I can't explain why this happens it just does.
Solution, Switch tanks
Hope these help you out.