A couple things:
1. Your fish are small and I would guess they're young. Most severums, especially young severums, take several tries to get the whole spawning thing right. I would let them keep trying.
2. You should leave some kind of light on for the fish at night. I've never had good experience with pairs who are tending eggs if you turn the lights completely out. I usually either give them a night light (a lamp or something next to the tank), or I leave one of the tank lights on. This allows them to see the eggs while tending them, and also chase away sneaky tankmates (like plecos).
3. If you want to try and see if the eggs are fertile, you can pull them and put them in a separate tank. You don't need an egg tumbler; just remove the item they were laid on (mine use the top of clay pleco caves) and put it in a bare-bottom 5 or 10 gallon tank with a strong airstone placed just above the eggs, a mini-heater, and a few drops of something like methylene blue to try and cut down on fungus. You can even go as far as to remove fungused eggs with sterilized tweezers if you don't want it to spread to good eggs, but this is probably unnecessary.
I used to pull angelfish eggs all the time and raise them this way. I have also done the same with Mesonauta spawns. Severums are no different. The eggs should hatch in 2 - 2.5 days at 82 - 84F degrees. If the eggs are not fertile, they should all be completely fungused over by the end of day 2 or 3.
Some people would suggest isolating the pair, but I am a firm believer that this is a bad idea. I've had cichlids turn on each other once the tankmates are removed from the spawning tank. The dithers seem to give them a target for their aggression. It also seems to help strengthen the parental instinct of protecting eggs/fry/territory. JMO.