Diagnosis
The hinge mechanism is binding. Grease and carbon buildup on the hinge pins or a misaligned lid.
How to Fix It
Let the grill cool completely. Wipe the stainless steel hinge pins on the body and the corresponding slots on the lid arm clean with a damp cloth. Apply a small amount of high-temperature dry lubricant (like graphite powder) to the pins. Work the lid open and closed 10-15 times to distribute it.
If That Didn't Work
The hinge springs may be over-tensioned or damaged. This requires removing the lid by extracting the hinge pins with a punch and mallet to inspect and possibly replace the spring assemblies.
Heads Up
Never use oil or standard WD-40 on the hinges. It will attract more grime and become a fire hazard. Use only lubricants rated above 500°F.