Match phototransistor to LED source
Your photo transistor is optimally responding to 600 nm.


White leds of 5000k tend to have their heighest peak in the blue range of 450 nm and a lower hump over 525 to 550 nm.


The setup is not ideal, but should still have some reasonable response, as long as you properly aim them at the light source. Both the transistor and the light source spectrum peaks are curves, and should work even if not directly matched, as long as the output is in the curve. Without knowing your specific lights spectral range, we cannot give you better guidance.