Visual effect anomaly

[Graham Samuel]

I have a rectangular area on a card which is used to cover a number 
of objects. The cover can be 'opened' or 'closed' via a button with 
this toggle script:

if the visible of cd grc "scPicShutter" is true then
     lock screen
     hide cd graphic "scPicShutter"
     unlock screen with visual effect Checkerboard fast
   else
     lock screen
     show cd graphic "scPicShutter"
     unlock screen with visual effect Checkerboard fast
   end if

The strange thing is that the closing ('show') always shows the 
visual effect, but the opening ('hide') never does. I can't see from 
the documentation why this should be so. Can anyone explain it, and 
suggest a workaround?

TIA

Graham
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          Graham Samuel / The Living Fossil Co. / UK & France