Hey there IMHO it could be a lock out , those rust spots are a sure sign of cal or mag def. Also check for violet or purple coloring on the leave stems. Also I would lower the ph to around 6.3 to 6.7. I find keeping the ph between these numbers help the plant to take up nutrients. And a good dose of cal-mag would help , and the good thing about cal mag is it's really hard to over do it.
!!! CHEERS!!!
@MrJones, soo sorry for the late reply.. missed your comment. I use distilled water with a bit of great white from time to time. Sometimes I'll add just a hint of guano but not often and if so I might use a saturating agent to help with uptake.