Often flushing urinals will block due to excessive limescale build up in the urinal pipe.

This occurs as limescale is insoluble in water and forms on the pipe surface. When the urinal is descaled the pipe is cleaned to the base of the trap, however there is often more build up past the trap which cannot be cleaned. As the microbes in the cubes start to remove the scale some of this partially degraded scale falls off the pipe wall and as it is soft, can block the drain.