Ya gotta feed 'em to make 'em perform. (nutrients and light)

Most MG products dilution rates are for a 10 day to 2 week schedule. If your's is a sativa dom, start with a 1/4 dose, and about 4-5 days later do the same. Watch for results. Once acclimated to the nutrients, increase 1/2 dose. I'd consider the label directions as being for a two week period, but I'd cut it in half, and give that 1/2 dose weekly. (rather than slamming a 2 week dose down their throats) Keep your eyes peeled for a good deal on quality, easier to use nutrients. MG works fine, but to some of us 'fine' isn't quite good enough.

When was the last time you flushed their medium?

Adding a few CFL's would be a benefit. The larger the plant, the more power (light) they need to finish.

Ph swings cause plant stress. Keep your ingoing water at 6.9 and you should be fine. Especially when using nutrients which tend to lower it more anyway. If using those color coded test kits, I'd properly ph your water, than add the nutrients. A tinted liquid gives skewed (unreliable) ph results. If this is how you determined your runoff ph, you are likely chasing an invalid runoff ph number.

Let us know if anything changes. Some sativa doms are slow finishers, so be patient.