From my experience I can see light burn being a good culprit. Is that DE lamps I see plotted around up high there? =) Considering you've gone this far without a hitch could be an indicator of that ,also nute burn/ph issues are something to definetly look into. If nothing has changed start looking at changed factors. New light bulbs, simple height adjustment, new water source, change in room temperature also effect DE output if the light doesn't run at proper temp. Could mean that you're getting a higher par output now. Etc... Strains are only so temperamental, we're talking about a weed here haha! Please follow up if you need more help!
