If they spawn once, they will spawn again. The trick is to mimic the same water conditions the previous breeder used, SG, Temp, light schedule, food, feeding times, and decor.
I can't tell you how many "deals" I have picked up because a person bought a set of proven spawners from someone and could not get them to spawn. I got a set of Black Photons (Sanjay variety) this way for $150.
The longest I have ever waited for a pair to start spawning again is 6 months.
For my clowns, 84-86 degrees, lights on @ 0700 and off at 10:30. No tank mates, pot or tile, and no substrate. Feed 0700, 1600, 2000 and 2130.
Your mileage may vary.