If you are struggling to get a temperature above 350F, there are a few things to check. First, ensure that the lower draft door and top vent cap are open enough to allow for air flow which will increase the tempurature.
Next, check your charcoal. Make sure there is plenty of lump charcoal and that it is not damp.
Finally, if those don’t help, make sure that the firebox has enough air flow and that the holes aren’t clogged up.
Be sure to check the accuracy of the temperature gauge in case that needs to be recalibrated