We tested one of these recently and it was suprisingly ok. Grow lights have a lot of red in the which is the opposite of what you want

https://www.amazon.com/QUANS-Blacklight-Fishing-Aquarium-Waterproof/dp/B07JFG6TL6

Is it a 2 pack emulsion, did you add the diazo sensitizer ? We recently tested the lamp below and it took about 6 mins with one of our diazo emulsions and this is with a better wavelength so you could be looking at 12+ minutes with the lamp you have

https://www.amazon.com/QUANS-Blacklight-Fishing-Aquarium-Waterproof/dp/B07JFG6TL6