Cannabis plants may be growing slowly indoors due to various factors. Insufficient or improper lighting, including inadequate intensity or incorrect light cycles, can hinder growth. Nutrient deficiencies, such as a lack of essential elements, may also contribute to slow growth. Inconsistent or improper temperature and humidity levels can negatively impact plant development. Poor air circulation and ventilation can create stagnant conditions that inhibit growth and increase the risk of pests and diseases. Overwatering or underwatering can cause stress and hinder growth as well. Additionally, the genetics of the cannabis strain being grown can influence growth speed.