Slow growth? What should I do?

As already said your light may be not strong enough or too far away however it could be as well quality of water you are using to mix your nutrients. If it’s tap water you are using check the PPM of plain water. If it’s above 200PPM switch to RO, distilled or de-ionised water. I had some issues with stunned growth on the beginning when using tap water. After changing the water plants got crazy in a good way. Hope that helps 🙌

Looks like lamp is too close

I think your lights might be too far from the plant or not strong enough to begin with... Also; light spectrum looks very blue...; blue light tends to decrease “internodal” space so maybe figure out if you can add an extra lightsource? Dont think this is a deficiency tbh, the growth looks healthy!