Thank you for your post.
Without seeing your file, it is difficult to determine what may be a possible cause.
If you create a new layout, is there any problem setting the tab order? If not, then your original layout may be corrupt. I would replace the fields on the layout that are having trouble and try setting the tab order again.
If you have trouble setting the tab order in a new layout, then either the file or FileMaker is not functioning properly. I would uninstall and reinstall FileMaker Pro and try again.
If you are still having difficulty, let me know, and I'll make arrangements to look at your file.