Diagnosis
This isn't a temperature problem, it's a technique and prep problem. The barrel may be too large for your hair type or you're not letting the hair cool on the barrel.
How to Fix It
Use smaller sections of hair. After the clamp releases and the barrel spins up, hold the hair taut for a full 10-15 seconds until you feel the barrel stop vibrating (meaning the hair has fully released and cooled). Then gently unwind. Prep with a light-hold mousse or heat protectant on damp hair, not dry.
If That Didn't Work
If the barrel genuinely doesn't feel very hot compared to when new, the heating element may be failing. There's no user fix for this.
Heads Up
This tool works best on hair that's about 80-90% dry, not bone dry. It needs a bit of moisture to set the curl.