2 lbs should have added about 55ppm. 3 more lbs. should have added an additional 80ppm.

But......... hold on. Those numbers were based on a volume of 4800 that you reported. I just now calculated your volume. I don't think you have 4800 gallons. I think you have more like 5600 gallons.

So, in 5600 gallons, 5 lbs. of stabilizer should have added 115ppm of CYA.

I don't know why you're not getting a reading. What kit are you using to test with?