>>We had a "not all of his sheep on count", which initially meant a sad or just downbeat person, i.e. as happy as a shepherd who lost a sheep. That somehow got mixed up with "missing a plank" (which later expanded into "missing a whole door", "missing a lumberyard") i.e. there's a hole and there's a draft through his head... so in the end you get "not all of his on count" (sheep or planks got omitted) to simply mean the guy is understaffed in the brains department.
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>A few americanisms I have heard for this meaning:
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>Lights are on, but no one's home.

Reminds me of "he's not alone" as expression for UI (not user interface, the other UI). We actually regularly see a guy around here, driving the bicycle down the back alley, always in merry conversation with someone, having great fun.

>Two beers short of a six-pack.
>Not the sharpest tack in the box.

"Dull as a grindstone"

>Not the brightest bulb on the tree. (refering to a Christmas Tree)

I've also heard of "not the sharpest knife in the drawer".

>Bats in the bellfry. (sp)

Now this one is just as picturesque as some of the others I now remember (some apply to me as well, as it took me so much time to remember :). One branch was based on "late ignition", and included any random reference to spark plugs, which are called "candlettes" - either to say they must be greasy and won't spark, or the wick is wet, not running on all cylinders, has drummed out (as in an engine in terminal stages of excessive noise before the transaxle breaks) and any other candle/motor related innuendo. One synthesis of all these is "push to start, blow to stop/turn off/extinguish" (the verb means "stop the flame" but applies to turning off anything that's on).