Thanks for the added details.
One common cause for concentration limitations is due to auto-shutoff.
However, and since this is not your case we must look at other factors to explain your results. With that said, the next point of interest would be the electrical characteristics of the generator itself - such as the voltage and current ratings under load.
Following this, I'd consider your configuration details such as; total amount of distilled water, electrode configuration; total wetted surface area and distance between electrodes.
Beyond this, here are a few ideas off the top that could explain your results;
1. Generator too weak to exceed given saturation(ppm)
2. Setup causing excess oxide production; lack-of stirrer, electrode hotspots or process breakdown(ref. no.1)
3. Exceeding maximum saturation(ppm) - causing precipitation
With that said, here are a few recommendations to help achieve higher concentrations;
a) build a more powerful generator using batteries and a current regulating diode
b) add a stirrer
c) use heat - to your raise the saturation limit and reduce oxide production
d) swap-out or clean cathodes during processing to avoid oxide buildup and precipitation
Beyond this, you could add voltage and current monitoring capabilities to help evaluate processing.
One such example of this can be found here: http://alternativetherapy.press/DiY_colloidal-silver-generator-setup_7.png
- hope this helps