Did you turn the tank upside down? I refill them all the time. Here is my method:
1) I weigh the empty tank with my food scale and write it on the 1 lb. bottle with a sharpie. Writing it on the lb. bottle means I only have to weigh it empty once. They generally weigh about about 14 oz. (I am not currently using my food scale for a diet plan so it is free to be used for this purpose )
2) Put the 1 lb. bottle in the freezer for about 30-45 minutes. I have a chest freezer in the garage so I am able to avoid taking them into the house.
3) Attach the adapter to a 20 lb. tank and turn it upside down. After getting the 1 lb. tank out of the freezer I attached it to the upside down 20 lb. tank and open the 20 lb tank valve for about a minute. After turning off the 20 lb. tank (be very careful here becasue with the 20 lb. tank upside down, righty is not tighty! I filled my garage with propane making this mistake ). That mercaptin smell hung around for a few hours even with my garage doors wide open.
4) I detach the 1 lb. bottle and weigh it. I do this until I put in 16 oz. of propane into the 1 lb. bottle. It usually takes me 3 iterations to get 16 oz. The total weight of the 1 lb. bottle is ~30 oz. Sometimes I have to put the 1 lb. bottle back in the freezer to encourage more propane transfer.
I feel like I have over filled them when I did not use the scale method. I filled two tanks until the pressures equalized one winter and when I looked at them in that next summer the tanks were deformed . That was the last time I refilled those ones!
We should all be aware that these Coleman style tanks are really only approved by the Department of Transportation (DOT) as single use cylinders. They are not designed by the manufacturer to withstand the fatigue of the fill/empty/fill pressure cycles. Personally, I am not worried as they have to have a huge factor of safety to carry flammable pressurized fuel, but the manufacturer declares it a single use cylinder to 1) make more money in selling more single use cylinders and 2) they don't have to go through a more rigorous testing regime to qualify it for a refillable pressure vessel. If you are doing this for your own private use none will be the wiser, but if you are doing it commercially watch out!
Sooner than later I am going to go the Flame King route. Buying those 1 lb. propane bottles cost a lot more money ounce for ounce vs. getting 20 lb. cylinders refilled. (Don't get me started on Blue Rhino type exchanges vs. going to a local farm store or local mom and pop gas station and getting your 20 lb. tank refilled...)
Fish are food, not friends!