Operation Guillotine Issues

So, I downloaded the game with my Playstation Plus membership, and it has worked great...until now. I get into the mission and everything goes just like it is supposed to. Then, after placing the mortar on the ground and firing the first shot, I'm supposed to be able to get over the wall with the help of one of my comrades. However, no matter what I do, he won't lift me over the wall...he just stands there and looks at me. How do I fix this issue so that I can actually play the game?