What exercises could I do to get my cob working properly from behind? I've tried slowing his trot right down to a near walk and then push for extended trot but it didn't do anything, any suggestions?
Get an instructor, as different things work for different circumstances and it depends on why your horse isn't working from behind. Rule out a physical problem first though.
Transitions, lots of them, especially rein back to walk, then eventually rein back to trot. But it is a long term aim, to strengthen his hinds, doesn't happen overnight like any other gymnastic exercise
Ditto wkwi - given some of your other posts, including the fact that you are 14, I would say a good instructor is the way to go. You're asking the right questions but there are so many reasons for the things you are asking about that you would get much more valuable feedback from someone on the ground with you. Are you a member of pony club? If not I would highly recommend it - good quality instruction, loads of support, and fun!