Not sure what else to do?

Hello, I'm about 2 weeks into my 2nd indoor grow, 15 days since potted to be exact, and I'm having issues with one plant. 3 out of my 4 plants look great and have grown vigorously, however the original harvests purple kush has not. This plant has always been slightly on the smaller side, but over the last 4 days its leaves have been drooped and growth has come to a complete halt or is seriously impeded at best.

The plants are in a 4x4 tent in 1 litter airpots and kept between 70-75°F (72.5°F average) with a relative humidity between 63-69% (65% average). The tent is well ventilated and has great airflow. They have been on an 18/6 light cycle under 1 Migro aray4 (250W) light at about 25% power. The amount of water they have received has varied a fair bit, but over the last 5 or so days I've given them 20-50mL of pH 6.5-7.0 H2O as needed.

The plant was initially overwatered i believe. Not by an absurd amount, but by enough that it took about 2 days before the soil showed signs of drying out. Just to be clear, the pot wasn't drenched to the point the roots were completely choked out, but the soil was definitely too wet. I initially thought this was the problem and have since administered more appropriate amounts of water. As the problem persisted I suspected the light intensity was too great. As of yesterday, I moved the plant under a Feit electric BR30 E26 (Medium) LED Grow Light Sunlight 60 Watt Equivalence bulb and have seen some improvement. After about 15hrs under the new light, the new growth (the top set of leaves) on the plant has perked up and is no longer drooping, however the prior leaves all look the same. Also some brand new growth has occurred with two little leaf nodes peeking out at the very top.

I concluded that out of the common problems with seedlings, overwatering or light intensity had to be the culprit, but after using better watering practices and about 15 hours under the new light and not seeing the plant revitalized, I decided to take it to the forums. I know it's a long post, so let me say thank you in advance for taking the time to read it. If anyone has any suggestions, comments, or advice on how to proceed, I would be very grateful. I will attach 2 pics. The 1st is with the leaves drooping and the 2nd Pic is about 15hrs after the plant was moved under the weaker bulb where the new growth (the top set of leaves) perked up and the new leaves have started to form.

 

Soil is happy frog ocean forest.

 

Yea, I have and do monitor the weight of the pot. What made me concerned was how vigorously the plant initially grew...then literally over night the drooping began and the growth slowed.
I have since made a few changes; reducing the amount of water used and changing the relative humidity from around 70% to 60%. The plant still has some droop, though after these changes it is much less pronounced. The growth is still quite slow, but I'm just going to continue on and see how things go.