The EC meter does not tell you 'all is well'.
The plants eat what they need, and leave behind what they don't.
After a while, unused fertilizer components build up in your res to the point where the EC can read in the desired range, but unbeknownst to you, there is virtually no USEABLE fertilizer left.

CHANGE YOUR RES! Your plants will thank you. Get on it, chop chop. Every 1-2 weeks do a full swap and guarantee yourself that the EC numbers actually MEAN something.