Patience is very important when it comes to working with the substrate spawners from Lake Tanganyika. Changing your water is good and that is not problem, but when you start playing with their substrate/sand and their shells, it can frustrate them. How often you clean your sand is going to depend on how deep it is and maybe your sand is too deep? I don't think you need it anything thicker then 1/2 - 1", that is more then plenty, and so it is easier to keep clean. These fish are quite small and the bio load that they are putting on a 20 gal tank is going to be minimal especially with 25% weekly water changes and a proper filter, you should be able to get away with vacuuming your sand less. When you do vacuum; becareful around the shells and try not to disturb them. The only time you want to disturb the shells them selves is if someone is missing, and then you can just smell the shell and know right away if there is an issue.
I have noticed issues when I move fish around, or change their tank around, it takes a while for them to settle in again. So keep your hands out of the tank!! And just let them be multies! Many people believe that it is live foods and clean water that will encourage your fish to spawn. You have covered the clean water, but what about foods what are you feeding them?? Some of the live foods you may want to consider would be: freshly hatched baby brine shrimp, daphnia, tubiflex worms, mosquito larvae, etc. These are the foods that will help put your multies in the mood!
Hang in there, these fish are born to breed, so it will only be a matter of time, before nature will take its course.