A sandwich short of a picnic

[natalie_1982]

Brick short of a load

What other ones are there that mean the same thing?

[shaneystar]

Tuppence short of a shilling.

[natalie_1982]

Not the full ticket

[HAnn521]

One playing card short of a deck...

[amron]

The lights are on but nobodys at home.

Head full of dominoes and there all chapping.

A few cans short of a six-pack.

[splange]

the wheel's still spinning but the hamsters dead.

[Peter Pedant]

few dolly mixture short of a full quarter pound {the fella in corrie]

not all there/out to lunch/out of it/off the wall

[Cetti]

A finger short of a mitten. Said tonight in Coronation Street by Vera Duckworth.

[cherrybomb]

A voucher short of a pop up toaster (Frank butcher)

A cog short of a moving part (my husband)

[Kay]

As daft as a brush with no head on.

[jacey]

The light's on, but no one's in

[LeedsRhinos]

A sandwich short of a picnic

[B00]

splange........as far as im concerned, yuo get the ****, that actually made me giggle out loud. :-)

[magabsb]

one floor short of a bungalow !

[Clare A]

Lancastrian: A bit behind the door (when the brains were given out).

Australian: A kangaroo loose in the top paddock.

[Xollob]

Some good ones from an earlier question here, and a couple of lists here and here. 

My favourite is "a few ears short of a bushel", or perhaps "couldn't pour pıss out of a boot with instructions on the heel."

[cazz218]

too much yardage between his goalposts

shes knitting with only one needle!

his telephone is off the hook

his belt doesn't go through all the loops

[Limeycat]

a snag (sausage) short of a barbie

[Philtaz]

A flash-to-bang time of 20 seconds.

[sweggie]

Brains of a rocking horse

If he ran a pub he'd forget to open