I'm looking back and I see 2 more potential issues:
1) you say you adjust the water before adding it to the res. Do you adjust the res solution after? You may need to. pH adjustment shoudl be the LAST thing you do before walking away from a freshly filled res.
2) Your city water is relatively soft. You should run calmag up to 250ppm before adding any nutes to the res, then add your nutes after, then adjust pH.