Is your route working? You can check this by opening the “Manage Lines” dialog and seeing if the route of the tram is shown as a coloured overlay of the same colour as your line colour. If you don’t see it, it is because at least one stop is unreachable. In the case of trams, this means there is either one stop on the line not connected to the tram network, or the tram-networks of each bus-stop are not connected. There may be a small gap somewhere along your line without a tram-line in the road.