For the first 2 weeks atleast you need to provide plants with 6000K+ or maybe 5600K. Hps you can turn on after it reaches 6 inches or more. Thats a no brainer .. Try a simple white LED bulb , you dont have to turn off the HPS just stick it next to current light.

I had it on 3500K warm white for 3-4 weeks same like you ,and nothing was happening.Switched out to a 20W LED bulb it was 7500K spectrum, and i got a massive plant in under 1 week.