Mick,
There are 2 possible reasons for stiff gearchanges. Firstly, the actual selector mechanism may be stiff, or secondly, the gears that should rotate freely on the main or layshafts aren't free enough.
In the first case, the problem often lies in the detent plunger at the front of the camplate - some of them had too small a flat on them, causing a partial hydraulic lock behind the plunger. Overfilling with oil can also make this problem worse.
If the gears seem to be selecting OK but the pedal doesn't go full travel, the problem is probably a gear that is not spinning freely enough to allow the dogs to align as you change gear. If the problem exists on all changes, it is probably the fit of the mainshaft into the sleeve gear. If the problem only exists when changing in and out of 1st, make sure the bronze bush that 1st gear on the mainshaft runs on hasn't been crushed by overtightening the mainshaft nut.
Also, make sure you have the correct match of main shaft and sleeve gear. There is a long bronze bush that the sleeve gear runs on - now Mr Douglas changed the design during the Mark series production run. I don't know which came first, but some gearboxes had the bush pressed into the sleeve gear with the mainshaft spinning inside it - others had the bush pressed onto the main shaft, with the sleeve gear spinning on the bush - I suspect this was the later version as it provided an increased bearing area. Mainshafts with the fixed bush had a groove down the shaft to allow the oil to run back from the blind end of the bearing - a mismatch would eliminate this drain, and may also result in incorrect clearances in the bearing - I would opt for a bearing on the slack side rather than too tight - on the Douglas, there are no side thrusts from primary chains, etc.
If you are new to 'Douglas', a change of riding style may help. Unlike most British bikes, the Douglas has an engine speed gearbox, and it takes a bit of getting used to. Personally, I only use the clutch for changing from 1st to 2nd - 2nd to 3rd and 3rd to top, I just roll the throttle off and change. Downward changes from top - 3rd - 2nd, I usually make with the throttle open and just a quick flick of the clutch as I change gear - as I say, it takes a bit of practice to get quick, clean changes.
Hope some of this helps,
Regards,
Eddie.