I see you have "finetaste" in fish! lol Biotodoma cupido is one of the nicest and cutest little eartheaters around. They have a reputation of being difficult to spawn and providing them with certain conditions seems to be the key to success. I don't have all the info about your tank so I will cover the basics for you. They are slow growers, this is usually a sign of taking a little longer to become sexually mature, so age is a question, and it maybe 18 - 24 months before they figure it all out. While they seem to be able to acclimate to higher pH, many believe that you need to have them in water that is around 6.5 pH, and temperature around 82-84 degrees F. They typically are found in pools with little or no water movement, so you want to make sure you water is diffused in the tank, you don't want any strong currents. They will want secure places to spawn, so you will need to provide caves as well as flat rocks, and they love a sandy substrate. A well planted tank will also be appreciated.
This is a shy subdominate species if there are other tankmates, it is likely you won't see a spawning, so you may want to consider a species only tank. This cichlid species amazingly doesn't fertilize the eggs at the same time they are being laid. The male is assigned to protecting the outer area, while the female lays her eggs and then she takes over security while he looks for the eggs to fertilize them. So having any other fish in the tank, they will get in there and break up the spawn. Catfish, pleco's etc. are famous for this.
There is a double whammy, because if you are lucky enough to get a spawn, the fry are extremely hard to raise. Getting viable fry is can be tough, but it appears to be a nutrition issue, live foods; freshly hatched baby brine is a must especially for the first few months, the fry are incredibly small and slow growing, and have small mouths so they are not able to take most conventional foods.
Speaking of live foods, I think this is another way to get you cupids in the mood. Offering tubifex worms; the black ones or the small red ones is an excellent food and will really help to get them in prime condition. Daphnia would also be another good live food for them. Other foods to consider are freeze dried krill and plankton. Most would agree that you are on the right track, raising up these fish together and allow them to pair off naturally. Once you have a pair, you will want to move them to smaller quarters, where you can control not only the water conditions, but food and if you get fry, you will be able to salvage them much easier in a smaller tank.
Don't give up yet, check your basics, the pH and temperature are critical, as well as foods.