Potency as a whole, is dependent on genetics and genetics only.
Poor potency levels from plants with high potency genes, can be attributed to a few factors - insufficient light strength/quality/amount, poor nutrition, stressed plants, heavily defoliated plants and harvesting either too early or too late, being some of the most common reasons for poor cannabinoid quality.
For a plant to be able to produce her best trichomes, she needs
to be grown in a manner which allows her to be as healthy as possible.
Inadequate/incorrect light is probably the main cause of poor cannabinoid development, followed by unhealthy plants. Heavy defoliation will cause a massive reduction in energy production levels, and this too can affect cannabinoid synthesis. Harvesting too late will produce sleepy/heavy "dope" that makes you lethargic, with very little !high". Harvesting too early will result in flowers with under developed cannabinoids, also producing very little effect other than a mild "buzz".
Auto flowering cannabis can also be not as strong as the same type of plant that is the photoperiod version. For example, one breeder offers a "FSP" strain in auto flowering with a THC level of 16%, whereas the photoperiod version is 22%.
Generally though, light quality and strength are the main reasons behind poor THC development in indoor plants. Cannabis needs strong light with unique light spectrums for proper photosynthesis, something cheap, low wattage "blurple" lights can't provide.
Hope this helps,..........
Organoman.