Plant grow problems

A photoperiod won't flower unless it receives 10+ hours of uninterrupted darkness. If you are certain this did not occur, that was not a photperiod plant. Somehow you got an autoflower seed. not enough info to help you... is that soilless or soil? etc... you just went from 150w to 200w, you mght be giving too much light. if there is no stem developing between the nodes (internode), ease back. This 200w light is probably best covering roughly .5m^2 - a tad larger is fine too but less than .6m^2. Over 18/6 you'd want 66-70% power hung to cover 70cmx70cm or a bit more, as stated before. When you get to 12/12, run at 100% from same hanging distance. Easy peasy. This plant might be too young for 'full' power, relative to hours of operation above. If you are giving more, definitely back off. Let the stetch guide the power adjustments. if you do full autoflowers, you can cover 150% of the space and run at 100% power on an 18/6 cycle. So .75-.80m^2 for autoflowers. If you run them in the same footprint as above for photos, run at 66-70% power. You should see that all of this is proportional to hours of operation. this should give you 35-40DLI with that fixture. If anything you may need a little extra hanging distance or 5-10% less power for any suggestion above.

It's due to incorrect VPD. In short, the external plant conditions (leaf temp, Relative humidity, and the temperature inside the tent/grow area). There is a specific range. Get yourself an Infared thermometer and you'll be all set. You can find the correct range at vpdchart.