I believe this is a bug.
This is probably happening because your project is set to completion date.
If the repeating interval is set to completion date and has a start date and a due date, the project or action will not forward the date.
Changing the repeating interval to assigned date will solve this problem.
However, if you don't do the action for a while, you'll need to continuously complete the project or action to catch it up.
Previous post:
http://forums.omnigroup.com/showthread.php?t=5507