Hi Tom.
I tryed your alternative macro and some other things, without complete success.
But I think that I have now the 'explanation'.
As of today, I used nvALT with the the option 'Hide Dock Icon' (in the prefs of the application).
Today, I tryed it without that option and...
tadam ... it works on both MBA & iMac ! 
I don't understand exactly Why but it works.
Thank you all for your help