This can happen if you create the song in GarageBand 2 and then try to open it in GarageBand 1 on another computer, because GarageBand 2 includes new plug-ins that are not available in GarageBand 1. However, this can also happen if a song references Jam Pack plug-ins that are not installed on the computer on which you're currently working.
For greater freedom, consider upgrading all the computers you use to the same version of GarageBand and install the same Jam Packs.