New wall hung toilet leak?

We are having a cloakroom fitted and a new Nuie wall hung toilet. The first basin was fitted but leaked. The fitters suggested it could be a small drip of glaze on the outlet pipe which was not allowing a good seal. Ordered a new toilet, same model, tried again but still same problem. Fitters ordered different seals, tried them but still getting leak. They have removed the glaze drip but still getting a small drip when the toilet is flushed. What else could the problem be? We were wondering if there is a misalignment, but the wall has already been tiled where the frame is. Any advice would assist.