TOO MUCH LIGHT/
Any type of stress on your cannabis plants, including many hours of exposure to direct sunlight without rest, can also halt or slow down growth. If you grow indoors and suspect light exposure to be the source of stress, decrease the intensity or move lamps further away from the canopy if possible. Know that seedlings are particularly sensitive to intense light.
NOT ENOUGH LIGHT/
Although requirements can vary from strain to strain, light is nonetheless a critical factor for the development of all cannabis plants. A lack of “good” light can absolutely lead to slowed growth. If you grow indoors and suspect that your plants aren’t getting enough light, try to
decrease the distancebetween your lamps and the tops of plants.
Another possibility is that you're stressing the plant by constantly bending. Seedlings are very sensitive to stress which will result in slowed/stunted growth.
PH maybe.
Genetics will also play a MAJOR role in its growth. What strain?