It helps if everything else in tank is very light colored as the fry are attracted to dark objects..
The pipe looks black? They don't feel the need to leave or no matter how dark the parents get it is not darker then the pipe ?
It took my discus 5-6 times to get it right so you may not be doing anything wrong and just need to let them get it right ?
I would pull the pipe and use a sponge filter and offer the pair a breeding cone to lay eggs on also ..