99 XJ only taking 1.5 gallons of coolant

Hi guys, doing a coolant flush and I'm only able to get in about 1.5 gallons of distilled water. I started the engine and let it run for a minute or so with the heater on but I really wasn't able to put much more in. I changed the thermostat while I was at it. Any ideas?

You still have old flush/coolant in the heater core.

If all you're doing it draining from the radiator or water pump, 6 quarts of coolant is about all it will take. There will be a bunch left in the heater core & engine.

Got it thanks. I also didn't let it run to temp (ran the heater full blast to circulate the fluid). Got a little more in after that. Going to let it sit, drain the water and refill with coolant. Thanks guys!