This problem can occur in scripts written in earlier versions of FD7 and FD6 / 5 because they don't place into the scene heading elements certain pieces of code Tagger needs to identify new revised text.
The workaround is to always open in Final Draft 7.1.3 any script you need to tag. It will probably be in Revision mode, so put the blinking cursor in a piece of text that's been marked revised, hit the Space bar and then hit Backspace. This will alter the file (making it eligible to be saved) but will not create any net change in the actual material. When you save the file FD7.1.3 will insert critical code Tagger needs to integrate new revisions properly. Now you can import the file into Tagger and it should work.
NOTE: All support for Final Draft 7 and older and Tagger 1 has been discontinued.