Need advise on flowering plants

I have my second batch of White Widows 4 weeks into flower and over the past week half of the plants have developed brown spots what appears to be like if they were real dusty but thats not the case. I'm starting to get a bit worried because it's getting on more leaves and getting higher up the plant. Any ideas?

spider mites?

spider mites leave a more white trail
I had them a week ago, got rid of them fairly quick though

look for small thin webs and eggs under leaves though, just in case
might be some kinda of other bug

No pests visable, inspected well, thought nute issue, ph issues?

To me, it looks like a case of severe nutrient burn. What nutes are you using, and at what strength and frequency are you feeding them?

Fert once a week on a light feeding schdule, water water fert, etc using Advanced Nutrients Sensi 1 Bloom with Big Bud supplement week 4 1200ppm total @ ph 6.1

1200ppm might be too much for your plants to handle. Every strain is different in how much nutrients it can safely handle, and you may have exceeded your plants threshold. You need to flush immediately. And I recommend getting a salt-leaching solution like Clearex to break-down the residual nutrient salts that have built up over time. They look very badly burned at this point and it may be too late to save them. But you gotta try, right?

I ran a flush, they've had just ph'd water for the last two feedings, so far the new growth and bud growth aren't showing any burn so i think it worked. It's def stunted them back a week plus right now.

rolla * is dead on. To me your plants do not like the 6.1 ph, soil bro ineeds to be 6.7 to slighlty higher. you have severe nute lockout and nute toxicity due to the low ph and high ppms. coupled with heat stress. This will leave the rust clolored brown spots pretty much starting at tip of plant and making its way to bottom-old foliage 1st, younger next.
This is very advanced stage and I think based on experience that your buds will stop being compact and start producing buds/flowers with no leaves and look somewhat abnormal. I posted on this 2-3 months ago.
the good news is studies have shown that stressing the plant can result in higher THC levels, and it turned out to be some pretty strong stuff, just going to look weird. harvesting at the right time will be crucial.
too much ppms, not enough res change, salt build up, burn from excellarated respiration leading to hot spots in the leaves-freckling-and burning. excessive temps accelrate the burning

4 weeks into it, forget the nutes for now and let her cleanse her system. or maybe just the basics at 1/4 strength. you will need some silicone to help rebuild her cells. pick up the stuff Rolla8 mentioned.raise the lights as well by about 1/2 foot or more for now.