Not sure why it won't go higher. have you tested straight nutrient to see what it reads? If concentrated nutrient reads 750 then you'll never get it higher than that.
BUT, if that is the case, I would suspect your meter is either faulty or not calibrated correctly.
BTW, tomatoes don't really need a concentration higher than 750 ppm.

Not sure how to test straight nute. I just have a cheap ebay tds meter

Is your nutrient in a powder form? If so, create a max saturation solution by adding nutrient to one cup of water until no more will dissolve (there would be some in the bottom of the cup)then check the ppm of that concentrate. that should give you the maximum available nutrient concentration.

Did you calibrate the meter when you got it?
Might start there.

I totally agree with "grizzman", tomatoes don't really need a higher concentration than 750 PPM. And the reading "electricblue" are getting on PPM meter would be not standardized correctly.