It would help if you told us what year/ model/engine size you are referring to as the design varies. That being said, sounds like your crank bolt threads or the threads in the crankshaft might be stripped. Mitsu has re-designed the crankshaft bolt for the V6 three times now, they are known to break when not torqued down correctly. To properly torque you need to use a special tool (can be home made) that has two stubs that fit into the pulley to hold it still while you apply the torque to the bolt. You can use a chain wrench for this as well but you need to be careful of damaging the rubber dampening material in the pulley if you use that tool. You need to remove the bolt and pulley and inspect and report back. Take pics. There are ways to fix this, even if you have broken the bolt. We have walked others through it on the wire.
The roll pin only makes sure the pulley is positioned correctly during assembly. It's the sandwiching pressure from the bolt torque that holds it in place and keeps it from spinning on the end of the crank sprocket. I strongly recommend anyone changing the timing belt of removing the crank bolt for any reason, replace it with updated version of the bolt and ensure to use the correct washer and also orientate the washer correctly. Its concave and needs the convex side pointed out.
