In my office it is our standard to create a whole new PDF to show our latest changes. To do this we have been using Adobe PDF, we combine all sheets into a single file, sheet size is set to whatever full size is for that project, and this process has worked well for us ever since we first started using revit. However, just the past month or so we have been encountering a problem where we get changes from our engineers, my drafter and/or I will make the changes in the model, one of us will create a PDF, and the PDF will not have the latest changes.
For example, this morning I created a new PDF for a project that my drafter was working on last week. I know he collaborated the file, I could see the changes he made in revit, but in the PDF that I created this morning, it did not have the changes he made last week, and not only that but the time stamp in the PDF said 12/10/2015.
Totally weird.
I close out of Revit, open it back up again, create a new PDF and PRESTO! All of the changes are showing up and the time stamp shows today's date.

Does anyone have any ideas as to what could be the cause of this problem?

Thanks!