Help needed with coal mine.

[Patrick]

I'm in act two and the mission is called Backup where you have to assist another government agent who is trying to infiltrate a coal mine for some reason. You have to climb a big tower overlooking the mine and the idea is that when the agent arrives at the mine you cover him with the sniper riffle while he goes about his talking and killing.
The problem I'm having is that when the agent arrives and biggins his mission none of my bullets are killing anyone,even the AP are having no effect. I seem to be out of range but the game wont allow me to choose my own snipping spot so I can't get closer even though the drone shows me better, closer snipping positions.I tried shooting at the red barrels that normally explode but some take multiple shots with AP and some don't explode at all, so I can't help this agent.
As a test I even started shooting at the baddies from the same tower before the agent arrived and I killed them all with no problem,but as soon as the agent arrives all the dead soldiers are no longer dead. The agent begins his move into the mine and gets quickly attacked while I'm snipping away but killing no one, he soon dies and the mission fails.
I can't even refuse to do this mission and move to the next one so I'm stuck.
Any ideas out there?
Maybe I've missed something so any help would be appreciated, either that or the game ends right there for me.

[CIG]

Before you hit the button for Wait for Agent it would be a really good idea to go ahead and do a Drone Sweep and Scan, and then remove the three Snipers from the equation if you can.
Sometimes the game will not allow you to easily detect the Snipers -- so you may need to use the Drone to reveal the one on the nest across the road from the entrance. Either way once you eliminate the third Sniper your JSOC Agent arrives.
What you do is follow their marker through the Mine Site - they will tell you if and when they want you to shoot and who they want you to shoot.

[Patrick]

Originally posted by CIG:

Before you hit the button for Wait for Agent it would be a really good idea to go ahead and do a Drone Sweep and Scan, and then remove the three Snipers from the equation if you can.
Sometimes the game will not allow you to easily detect the Snipers -- so you may need to use the Drone to reveal the one on the nest across the road from the entrance. Either way once you eliminate the third Sniper your JSOC Agent arrives.
What you do is follow their marker through the Mine Site - they will tell you if and when they want you to shoot and who they want you to shoot.

Thanks for the advice CIG, I'll try that when I get around to playing it later this week.