I blacked out my tank once for two weeks for velvet, and the lily grew 3 new pads. There is probably some unknowable qualitt to you water, like to much boron or something weird. I see it all the time where people cant grow one plant for some reason. Someone asked me the secret to growing dwarf sag, since it grows ceazy in my tanks , when i. Reality its one of like 5 plants i can grow consistently.
Maybe you've been unlucky and have gottem bad bulbs, or maybe theyve been outcompeted by other plants(although this doesnt really make much sense). There are a couple trickly lick flipping it over or removing it and leaving it moist, out of water foe a month that may be worth trying out.