i use GIMPshop & i'd imagine it's close to GIMP 2 so i'll hazard a couple of guesses...
check the layers - are they set as "replace" or "combine"?
with "replace" your animation will delete the old layer & start afresh with the next layer - with "combine" the layers will just be placed over each other so any transparency will allow the previous layer to show through
maybe the layer is "locked"?
this is a function i don't fiddle with on mine - but make sure the layer with the bubble on isn't "locked" (it may have a padlock symbol by the side - it does in GIMPshop)
if the .xcf file isn't too big - open an account at http://www.fileden.com & upload it there - i'll download it & have a look for you dude