The only way an expansion tank can drain is for air behind the bladder to push it out. If your tank does not drain completely than it does not have enough air in it to displace the water inside. You should be able to easily move the tank empty.
To drain an expansion tank
First you need to drain the tank as far as it can go by itself. Next using a bicycle tire pump fill up the tank only until the water begins to flow out. Do not over pump the tank or you may ruin the bladder. Keep doing this until you are sure all of the air is out of the tank. Now to obtain the proper air pressure this can only be checked with the tank empty. Fill the tank up with 2 psi below the start of your pressure switch. (Example 30 psi on means 28 psi in the tank.) If the tank will not hold this pressure then the bladder has failed.
Now it would be a good time to disinfect the tank with some bleach. Use an empty filter sump housing to introduce about a 1/4 cup of bleach into the tank you will need some valves to do this. Now rinse the tank by filling it and draining it several times.