Forum:I cant turn in missions after i do them or do any missions in some places, anyone have a clue what may be wrong??????

I cant turn in missions after i do them. An some places when i do the mission i go to the spot where i suppose to be in the mission and it wont do anything. I will say kill so and so squa and i do and go to turn the lever but it wont let me and i will be exactly where it tells me to be at. anyone have a idea of it