It’s 150mm 3-4 years old with a ZTTO lever. It seems to work perfectly. Extends quickly as soon as lever pressed but fails to go up about the last 30mm.
Cable is fine, moves freely, long enough to allow full extension. I can’t find a valve so assume this is one of earlier ones with sealed innards. An
y bright ideas why it won’t extend fully without lifting the seat?

Definitely no valve under a black cap at the top of the unit under the lower seat clamp, mate?

Have you took the post out of the bike and operated it by hand to see if it still happens? (Push the actuator on the bottom) If it works fine it’s most likely the cable.
If it doesn’t work then it’s most likely the cartridge.

All of the earlier brand x posts had a valve under the seat clamp, there will be a rubber bung over it.

It’s the newer posts which are sealed units.

Seatpost clamp too tight?

These posts are really easy to strip, inspect, grease… check out a few YouTube videos. Replacement parts are pretty cheap and easy to find.

I thought the older ones weren’t sealed, as above remove the saddle and there should be a rubber grommet thing that hides the valve.

+1 take it apart and clean it.

Then if still not working, investigate further.

Also a new cable is a cheap fix and often solves the problem.

Aaargh!?!? It was the air pressure after all but it took some time to confirm because I hadn’t realised my shock pump had given up the ghost. Instead of removing the saddle and adding pressure, I ended up stripping it all down, putting on a new cable and it still wasn’t working. Luckily my son had left his shock pump in my garage so I tried that and hey presto, it worked beautifully.
At least I shouldn’t have to touch it for a good while.
Thanks to all for help and suggestions