I dealt with this when I purchased mine back in March when this item was first being sold at a discount, as did a few others.
I ended up finding out that the connectors for the phone on the back rail of the lapdock do not stick out far enough to make a good connection. If you're comfortable with basic do-it-yourself jobs, you need to open up that back part where the connectors are. I ended up unwrapping and removing a piece of metallic tape that went around both connectors and seemed to get bunched up in the hole where they were supposed to come through and connect to the phone. With that tape out of the way, apparently I got them to stick through a fraction of an inch further because I never had the problem again.
This might not be your problem, but there were a good number of people who have had this issue with their lapdock. I remember a bunch of us bought them when they first went on discount and then there were a good handful of us posting here and on other forums, bummed out that ours weren't working.
I ended up figuring this out when I realized pushing down on the phone with a lot of pressure into the lapdock would make it connect for a few seconds, and then once I let go, it lost connection again. So try that with your phone, really press it down into the lapdock seat, and if it finally starts working, you likely have connectors that aren't sticking out far enough as well. Of course make sure you don't have any case or protector on the phone though, as that would create the same problem. Same goes for extended battery, some people could get the lapdock to work with the big battery attached, others couldn't.
