I'm with Bill on this. There are spring loaded plungers on the gear selector and camplate. Spring loaded to return these items ready for the next gear selection. A seized plunger on standing or a failed return spring is a likely cause of your malady. A good run if possible, or prolonged idling to get engine and box nice and hot, could be all you need to help free off if seized. Otherwise it's pick and shovel time and the joys of unit construction, 'cos it is a whole lot more complicated than the A10 box, seems they build the bike round these plungers.
More information on Brit Bike Forum, just internet search under A65 Gear Selection Problems. Sorry folks, I am not too cute at posting direct links. Sump Magazine (online only) has a bit of background on A65 Clutch set up, should you need it.
As to the clunk into first gear from cold, the trick was to kick it over with the clutch lever pulled back, before turning on the ignition. This would free off the plates, and avoided the loud rattles and clunks when the input shaft remained driven by drag but rarely proved effective at completely stopping the clunk, as the gearbox input shaft still crept round with little to hold it still. The design probably needs this amount of initial movement to get the drive dogs aligned to start with. Once under way the simplicity of a constant mesh box and groaning layshaft bearings are there to enjoy. At least you knew first gear had gone in, before making a fool of yourself.
Swarfy.