The two key needs for any hot fire are fuel (charcoal/wood) and air.  If your fire isn’t getting enough of either of these, you won’t be able to reach higher temperatures.  You can easily achieve 400+ F on the indirect side with the right set up.  A couple tips:

1. Start with 1/2 to full chimney of fully lit charcoal, depending on how hot and how long you’re going to cook.  Small piles of “starter” coals like what is used for low ‘n slow heat will never get your grill up to temperature.
2. Open up your vents, and more importantly, make sure they aren’t clogged with ash.
3. Keep your charcoal dry.  If stored outside, or in a very humid environment, it may not burn as well as dry briquets.