Flood and Drain - Jack's 321 - having problems.

I'm doing flood and drain in buckets with rockwool. I started using Jack's 321 because I heard good things about it. Everything was going great until about 5 weeks into flower.
My PPM after mixing a fresh rez is about 1350. After a feeding the rez would shoot up to around 1700. I'm wondering if that could be caused by a salt buildup?

Anyway, I guess I did not think it was a huge problem, but over the next week or so the plants were showing signs of nute burn.

What would cause the ppm to rise like that?

Well if you have a salt buildup in the medium that can cause that to happen.

What medium?

Are the plants in pots? If so pull one, put a tray under it, pour some quantified (PPM and pH) tap water through (like a mini flush) and compare the PPM of the water after. If it's rising a lot then you definitely have salt buildups.

Yea rockwool can have salt build up its why my guru taught me to always do a 24 hour flush when changing res before adding nutes. Also I recently switched to Jacks RO and it has made things a hell of a lot easier and my plants love it.