visual effect, tabbed btn, groups

[sims]

  I have a tab btn with the following script:



  on menuPick newTab,oldTab
     lock screen
     hide grp  oldTab
     show grp newTab
     unlock screen with  visual effect dissolve 
   end menuPick

This shows one group and hides the other (actually two images right now).
Works but no visual effect.

Should the visual effect work with 2 groups?

ciao,
sims