If it works in wood but not metal your cutting forces may be too great for the machine, things you could try in no particular order are -
Reducing your feed per flute.
Clearing the chips with air.
A light cutting lube intermittently sprayed on the work, WD-40 or a 20/80 mix of engine oil/paraffin.
Doing the job in multiple passes, getting just a little deeper each time rather than a single full depth cut.
Regards,
Nick