Your paddle is most likely stuck for one of two reasons: dried salt around the clamp causing it to seize up, or as a result of silt building up and drying inside the shaft. This can be caused by sand or grit in between the two sections, which has been ground into a paste by adjusting the sections.
The best way to approach this is with two or three people. Face each other on different sides of the paddle. Take hold of the top and middle sections and twist in opposite directions - go back and forth to break down the build-up of silt. Once there is some side to side movement, you can then try to twist and pull, the bond should have now broken and you should be able to separate the two parts
We made a quick video of ourselves doing this at our HQ recently, which may help explain better how to do this and what level of force it will require to clear the grit.
It is very important that you thoroughly clean the extension and the inside of the paddle shaft once you have successfully separated them.