Workflow create item intermittently not adding date

I have a workflow on a task list. What it does is take an item and then create a new item based off the original. The new due date is calculated and assigned in the workflow. So if the task is marked as annual, I add one year to the original due date and then assign that new date. Same with monthly, quarterly, etc. The problem is coming up that sometimes that date isn't being assigned. I can't tell for certain, but it seems kind of random of when the date is being assigned and when it isn't. I'm not sure what the issue could be. Any help would be appreciated.