Coolant flush

I just fixed my thermostat and I wanted to do a complete flush of the coolant system. I drained the radiator overnight. Then I filled the reservoir with product to flush the system. I added about 1.75 gallons of water. I stopped there because the reservoir was overfilling at this point. I bled the line from the bleed holem. But for some reason the reservoir stays full. I ran the car for about 8 minutes and the water was squirting out from the top hose of the reservoir.

Then I attempted to drain the radiator but nothing came out. I can hear water in the hose that goes to radiator and from the radiator if I sneeze them.

How come nothing's coming out when I drain it? I should let you know that before I started this I was able to drain the radiator overnight just the night before.