We had this same problem with our dog when we moved to Germany and had to live in an apartment. We've had Buddy his whole life and he was 5 yrs old when we moved here to Germany. His first 5 yrs he had a big fenced in yard to use. But when we moved here we had to get him use to using the bathroom on a leash. He didnt like it at first. I took him out several times with no luck. So finally I decided to give him lots n lots of water until I knew he couldn't hold it anymore. When I took him out it still took a long time before he finally pee'd. When he did I kinda dabbed the urine with a paper towel and then took the paper towel to an area I wanted him to use the bathroom in. I then rubbed it on a tree to get his scent in that area. From then on I've had no problems with him not wanting to go while on a leash. I also taught him and my other dog to go on command by saying go pee etc while they were already doing it. I taught them this because after growing up in the south where the winters are not real winters and then moving to GERMANY where it feels like Alaska...I wasn't too fond of walking around for so long waiting on them to go. haha Now they go as soon as we get to their area. Good luck!