Thank you for your post.
These type of issues are frustrating (for both the user and us) because they are not easily reproducible.
I did find a post that may be related. See:
In that case, the Status Toolbar does not update after Refresh Window while a script is running.
When this does occur, is it usually after a long time using FileMaker Pro (meaning possible memory issue)? Does this usually occur after running the same script or script trigger? Any other information you can provide may be helpful.