Nice fish and photos..
Here are some things I might try.Apoligize if anything is obvious to you or redundant.Thats a big tank but contains many fish that could make the male nervous and distracted.It works the other way too, establishing and keeping a bond w/your Severums.
1-Ditto on Ned's posts(obvious)
2-After fish are conditioned,do a safe water change that will not drasticaly effect your P.H. Vacum the gravel well anticipating pits being dug for wigglers etc.
3-Some fish prefer to spawn at different times of the day.I'd try and do what you can for a water change several hours before the tank lights are shut off.Leave room lighting on for several hours after.Give fish privacy.
4-Provide several possible spawning sites.
5-**Make some semi-sheltered spawning sites so the fish(male) are more secure.Use the rear,corner of the tank.Put a large rock(slate) flat on the gravel and have it hidden behind a big,tall piece of driftwood and plants.Or a large flower pot on its side w/a piece of slate at it opening and again something in front of slate for privacy.Large PVC pipe.They are not cave spawners but will feel more secure knowing there is shelter for themselves and fry.
6-Raise temperature up a little to a safe level.
7-Keep reading up on them and try different things.
8-After all this babbling,the male may just be immature.I bet they will produce fry. Good Luck