Neolamprologus helianthus is a fun fish to work with, but they can be tough to spawn. Or least they have been for me! Its hard because in a smaller tank one on one, they like to trash each other, and in a community tank, it appears they are spawning, but you never get any fry just like you are seeing. One issue is that they do not have large spawns, 25 - 50 has been typical for me.
I would say they do not like the shell. They tend to want a more secure site, and spawning caves or inverted flower pots seem to be more readily taken too. They will tend the fry up to a certain point in a community tank but once they know they can't contain the fry, they will eat them. Its the attitude that if the fry are going to be eaten, they would rather do it themselves.
I am doing them a couple of different ways, one is in 30 gal aquarium with others, like 8 - 10 in there, and seem to have a couple pairs forming, but the only way to get the fry out is to remove the fry before they freeswim, and so I have to pay attention. I also have a pair in with a group of Aulonocara Jacobfreibergi and this seems to be working better, they pretty much ignore each other, but the male still has to defend the spawning site which keeps him busy. I still have to remove the fry before they freeswim, but it is easier to detect that they have spawned.
Bottom line don't give up, move them to a smaller community setting if you can and if that doesn't work, keep going smaller.