Are you using both A, B parts of Heavy?
What is the warmest temp you think the grow space gets too when the lights are on, and how long a period does it stay that warm?
You have some leaf curl that can indicate air temp to warm also with distinct brown patchiness which also tends to indicate air temp too warm. You're using an MH and purple LED, correct? If you have warm lights on during the heat of the day and this is early September, your temps might be too warm.
If the fert is of good quality, you probably don't have a nutrient issue. Looks like they've grown pretty good so far. I suspect that 5.5 might be too low as I think 5.8 or 5.9 when nutrients are fresh is better. pH will drift upward as ppm goes down as it's the ppm value (the nutrients) that really drive the pH value. People seem to think it's the other way around but that's not the case. You can can't force plants to do better by altering pH. You have to get the nutrients right and pH will take care of itself. A good idea is to read info on the container, on the web about the ferts, or contact the company directly for advice on ideal ppm levels and how to handle the pH issue.
