I just recently flushed the radiator and came across a problem just like yours. It's called air pockets or air bubbles. What you have to do, is make sure that reservoir tank is lifted up. Takes a couple people to make it happen. 1st person lifts the reservoir as high as he/she can hold it up and the 2nd person starts the car and all the other fun stuff. Now... disconnect the 2 small tubes that connects back to the reservoir tank, and put em inside the air box, don't worry theres holes on the bottom of the air box so the coolant n stuff can drop into. That really didn't work for me... Everytime I poured coolant in it, it would go out that way, so I what I had to do is PLUGGED one of the tubes and set the other FACING up. You will know your car is taking the coolant in is when the level on the reservoir tank dips much much lower. This is the time to put more in.
Hint: I use a couple of stretchy ropes, I forgot the name of em, but I call em Bungy ropes, and I used that to hang the reservoir tank from the Hood
. Remember, the higher the tank is, the better/faster you can get the air bubbles out. The little butterfly valve screw next to where you pour the coolant needs to be unscrewed too. Remember to rev it a couple times and turn the heater on a few times to let it circulate. When you are done, make sure you put back the little butterfly valve screw and check the coolant level.
Hope that helps =)