First thing to recognize is that you do not have a big problem. Yes, it can be fixed as soon as you can identify the root cause.
Plants can 'not take up' minerals for a multitude of reasons, and not having enough food is just one of them. I do not think it is time to make nutrient changes (increase) at this point. You plant is starting to flower so both Phosphorous and Potassium requirements are getting higher. Instead of increasing the amount of food, I would check to see if your nutrient company recommends a slightly different formula for flowering plants that increases the percentage of P and K while reducing the concentration of nitrogen. (too much nitrogen will produce very dark green leaves with tips that 'claw' down).
I think that getting some distance from the light (I am assuming that you have done all you can) is one step, and if you can get the temperature into the 70's that would eliminate another possible problem.
With the plant looking to be generally in good shape, I am thinking that environment is the most likely issue. That being said, PH is always a concern and PPM of your runoff water counts too.
There may not be a 'pat' answer to your problem, but you are on it early, and don't quit asking questions until it is fixed.