Well you could try what the script below does on your camera. It's supposed to press the shutter once, waiting an optimal time for half-shuttering (much like the "shoot" command would do). If it doesn't, there's something wrong (or I'm mistaken, I didn't test this test script on my camera yet). And if replacing 206 with 205 helps, your camera follows the Digic II propcase set, at least in this respect.
rem digic III get_prop 206 test
@title test1
@param a delay1 (*10ms)
@default a 1
@param b delay2 (*10ms)
@default b 1
a=a*10
b=b*10
press "shoot_half"
sleep a
do
get_prop 206 P
until P=1
sleep b
press "shoot_full"
sleep 300
release "shoot_full"
end
If it works, I'm sort of puzzled. Try removing all sleep statements from the above test script.
If the script doesn't work, propcase 206 probably never gets set "1" on your camera like it does on mine. You can try increasing parameters a and b in case increased delays help. You can also add sleep statements anywhere in the script you like to see if they help. You could also try experimenting what property 206 actually does on your camera. Go to CHDK debug menu and enable showing propcase page 20 to see how property 206 changes when you press your shutter in various ways and modes.
To find out how it works during the script run, you can add commands such as
get_prop 206 P
print "prop206=",P
to your scripts to figure things out.
One more thing... there's probably an easy quick&dirty workaround to your problem. That fast shoot get_prop 206 loop (before md_detect_motion) isn't necessary for fast MD to work. It is essential in making it work smoothly in all kinds of different situations, but if it doesn't work for you, you can replace it with a (preferably configurable) delay. Usually something like sleep 3000 will be more than enough (but not always... Tv, ISO, scene brightness, jpg/raw saving time etc. may affect this). Too short a delay and you may end up in an endless motion detect loop with the camera detecting motion and shooting because the LCD image is still settling and adapting when md_detect_motion is already armed. But at least you'd have access to the fast MD feature before someone figures out what's really wrong here.