Coolant not getting sucked into radiator

What is your question?

I was flushing my radiator, starting with just a distilled water run before draining and adding BlueDevil radiator flush with water and running again to operating temp. My cars coolant capacity is roughly 3 gallons, but yet each time I could barely even fit one gallon of water in there. I watched some videos of people flushing radiators and saw that when they turned the car on, it would suck in the fluid so they could add more. I did this, the reservoir did not go down even the slightest I decided to check for air in the system via the heater, I notice the car is taking much longer to heat up but even after having the heater on full for a while, it never got as hot as it should on full heat. So I flushed the car again with just distilled water with the cap off to get the air out. Didn't change anything I inspected the hoses, I see no apparent issue I noticed that he drained out coolant and fluid was pretty nasty, maybe just clogged? The car has never overheated or anything