Stuck on allied mission
 

The mission that starts out were you defend the base that is loaded with tanks. The germans initially attack from the north but after a while you get reinforcements and they attack from the east. Once you've repelled both you have to capture their outpost and eliminate the mortars. I have killed every enemy on the map but the eliminate mortars objective will not complete. I've tried shooting at the mortars themselves with tanks. Tossed anti tank grenades at them but I cannot destroy them so I think that the mission is bugged for me. Any insight?

Have you eliminated all the howitzers too?