If you create a script like this -

when green flag clicked
forever
set [variable v] to (pick random (0) to (100))

And then try to hide/show to variable using the check box, it just doesn't let you sometimes.
BUG

This glitch is likely due to a high volume of requests being implemented on the server.
You can easily fix this with a wait 0 seconds block.
Here is an example.

when green flag clicked
forever
show variable [v v]
wait (0) secs
set [v v] to (pick random (1) to (100))
hide variable [v v]
end

The wait 0 seconds block gives it a very, very short pause (so short that the viewer will not be able to tell) to allow it to process the current commands and carry them out.

Hope this helped : )