Things such as contaminated calibration standards and rinse water, and your probe storage can cause your EC/TDS probe to not calibrate.
Remember that the calibration standards have no buffering capacity. Try taking some new/clean beakers and pouring fresh standard into each. Then, use one beaker as an additional “rinse” to help keep the standard for calibrating as free of contamination as possible.
Contaminated Rinse Water
EC probes are very sensitive, and as mentioned above, the calibration standards are easily contaminated. Old rinse water, or water from the tap, can introduce contaminants. Distilled water can be used, with deionized water being the best option prior to calibration and post calibration. Calibrate from your lowest concentration to your highest concentration standard, rinse in your standards not in deionized water when still in calibration mode to minimize contamination.
Storage of the Probe
EC probes should be stored dry unless they are a combination EC/pH probe. If you have a combination probe, the probe should be stored in HI70300 storage solution in order to keep the pH glass probe hydrated and in working order. If you have a combination probe, make sure to rinse the probe really well with deionized water when you remove it from storage solution. The storage solution is very salty and the salts will contaminate your EC standards if you do not rinse the probe off.