Well in the first picture in the morning you took, she looks REALLY dark green, but the ones in the sunlight seem a tiny bit lighter. Could be you are overdoing it on the nitrogen...it would make the leaves appear to be extremely dark green and would cause the stems and leaf petioles to become weak/droop like you are experiencing. I am not at all familiar with Sunshine mix as we don't have that brand where I live. Is that a prefertilized mix ?
EDIT:
Ah I went back and read your first post. Nitrogen overdose would also cause the plant to uptake water slower than usual as well as cause the plant to grow slower than usual. And are the leaves waxy looking? They look that way from the first picture
If I were in your position, I would get a nice spray bottle and mist that plant about an hour or two before the sun rises. This will help her leech out some nitrogen. Just be sure not to do this once flowering starts, as it could promote mold to mist the plant. I wouldn't advise flushing out the soil with a lot of water unless you are sure that it will drain and not create a miniature pond, lol.
Your other plants are the perfect color that you should shoot for.