Figured it out!
The achievements screen is accessible from the main menu, which also includes the typical "continue" and "new game" options. Variables are restored only when the player selects "continue," so none of the achievement tracking variables were active (sorry, I don't know the correct terminology - not a programmer).
I fixed the issue by including the following on my title screen:
on load do
restore "achievement1"
restore "achievement2"
...
end
I hope this helps someone else! store/restore has been the steepest learning curve for me.