I'm going to strongly recommend here on record that you try a nute that is buffered for soil use, to further keep it in the desired range. The Canna line is an example of one.

Also, you have been doing this a while; if your electrode is older, it may actually need cleaning. This is what I do at teh lab for glass electrodes:
Acetone will remove organic gunk, and 'shocking' it by placing it in a solution of 50% pH down for ~30 seconds, RINSING WELL, placing in 50% pH up for 30 seconds, rinse again, repeat 3x, always ENDING with the pH down, then rinse off REALLY well before use- this conditions the electrode.
Also even if it doesn't need cal, check it against a fert solution that you have tested with liquid drops.