On a transponder or remote head key, three things have to be right for a replacement to work: the blade has to be cut to the locks, the chip has to be programmed so the immobilizer lets the engine start, and on remote head keys the buttons have to be paired with the vehicle.
These are usually done in one visit. A copy cut at a hardware counter without the chip programmed will open the doors and turn in the ignition, but the engine will crank and stop or not start at all. That is the immobilizer working as designed.
If the original key has worn, a new key cut from the key code is often better than copying the worn one. Worn keys are a common cause of a Mitsubishi ignition that is hard to turn. See worn keys and poor key copies.