PSU not providing enough current
The power supply's job is to supply 12VDC regulated- it's up to the Peltier to draw enough current at that voltage. 

Assuming your power supply voltage is actually 12V (and assuming that you have not put something in series like an ammeter that would excessively drop the voltage)-- so the voltage across the Peltier device is actually 12V +/- not much, the Peltier simply isn't drawing the current you think it should. 

Perhaps the Peltier is designed to operate on a 13.8V automotive "12V".