DIAGNOSIS This is a classic symptom of a dirty grill, specifically ash buildup in the firepot or a clogged drip tray/diffuser plate choking off airflow. The fire can't get enough oxygen to burn hot enough.
FIX Do a deep clean. 1) Cool the grill. 2) Remove the grates, drip tray, and the diffuser plate. 3) Vacuum out ALL ash from the entire bottom of the grill, focusing on the firepot. Scrape it clean. 4) Check the drip tray liner—if it's a foil liner, replace it. If it's the stock tray, scrub it. Grease blockage there restricts heat flow. 5) Ensure the diffuser plate is seated correctly, not blocking the firepot. A clean grill is a hot grill.
IF THAT DIDN'T WORK The problem could be your pellets. Some 'blend' or fruitwood pellets have a lower BTU output and simply won't achieve the same max temp as pure hickory or oak. Try switching to a premium brand of 100% hardwood pellets known for high heat.
HEADS UP Ambient temperature matters. In very cold or windy weather, the grill will struggle to reach and hold its highest temperatures. It's working against significant heat loss. Using a grill blanket or moving it to a sheltered spot can help.