My question is in regard to HAWX for the xbox 360.

I completed Mission 19, Monarch, on all three difficulties in co-op mode. I methodically went thru each of the 18 other misisons to complete each difficulty level.

However, the achievements for completing the campaign on each difficulty setting is not registered in my gamertag. I received 500 points for completing the mission on Normal difficulty yet 0 points for Hard and Expert.

Has anyone else experienced the same problem?

Sounds to me like you're on an Xbox 360. Have you tried playing through the levels in single player campaign only ... On the PC and the PS3, playing through the entire campaign (19 operations) in elite only gives the points for normal, hard, and elite.

It may be that the only way to get credit for completing the missions in hard and elite is to play single player campaign instead of multiplayer coop