Diagnosis
The Tailgater is a smaller grill, so maintaining high heat can be a challenge, but this usually points to one of three things: a dirty temperature probe, poor pellet quality, or ambient conditions.
How to Fix It
First, clean the RTD temperature probe. It's that thin, silver rod sticking into the cooking chamber on the left side. After the grill is completely cool, use a bit of fine steel wool or a damp scrubbing pad to gently scrub all the carbon and grease off the metal probe. A coated probe gives false low readings, causing the controller to under-fire. After cleaning, try your high-temp test again.
If That Didn't Work
Check your pellets. Low-quality pellets with too much filler (sawdust) or pellets that have absorbed moisture won't burn hot enough. Make sure you're using 100% hardwood Traeger or another reputable brand, and store them in a sealed container. Also, high heat is harder to achieve on very cold or windy days. Try positioning the grill out of the wind.
Heads Up
The 'High' setting on the Tailgater is designed to get you to about 450°F, but real-world results vary. Consistently missing your target by 50+ degrees after a probe cleaning points to a failing controller or a weak induction fan, which are less common but possible fixes.