Hm. Can you just confirm that each of the action groups are set to parallel? (Control-click > Parallel)
If they are, then it's certainly possible there's a problem with the data. You could try recreating the project or the action group and see if it persists.
If they are, then it's certainly possible there's a problem with the data. You could try recreating the project or the action group and see if it persists.