This is common and I cannot for sure cite the reason but I think it is because new versions of ArcGIS are not 100% compatible with old MXD's
There are three things you can do that typically solve the issue. Even if you save an MXD with a new version it still can be an issue.
Try running MXD doctor on the MXD's. when you run it have it create new MXD's
This typically works.
Create brand new MXD's from the ground up. Least desirable as it consumes a lot of time.
On the other hand you can also create a new mxd by grouping all your features and then saving the group as a Layer file. Then from a blank new session paste the layer files.
However, this is what I do and it works.
From your MXD go to the layout view. Select all the elements and then COPY.
Start a new session of ArcGIS. Go to the layout view. Grab the data frame and move it off to the side. Resize the paper as to what you want and paste the copied elements. Save the MXD.
To me this is best as it will not only transfer your features, but it will transfer your labels and you layout.