When i was switching over my lever the other day I ran across a few exploded diagrams, I only have the lever one to hand but sure there was a decent one around for the post too. And suggestions were that they weren't that complicated to fettle.
You'll be much better at diagnosing them than I am but it must be something stopping whatever it is that locks the stanchion in place. Either the lever isn't fully returning to extended, air in the system expanding like Andy says (i'd be surprised at this as I don't think the air would expand to give enough force), too much air pressure in the post itself maybe highering the force it needs to clamp or, the most likely, is going to be something along the lines of something sticking in how the stanchion is clamped in place. For which I have no experience.
At worst, a cable tie (re-useable one if you're really fancy) will get you a seatpost that you can at least sit on and have stay in place to come riding tonight!
