If you do a search for "ILS capture" in the 727 Captain forum, you will probably find the answer (make sure to select "all" in the This post was made in the last... section). There are also a number of posts that will give you advice on how best to capture the ILS/GS as well.
From my search, it seems that if people were using GPS tracking (with the NAV/GPS switch), it won't capture the ILS, so they needed to make sure it was switched to NAV mode to capture it. From memory, I did it a little too often way back then. :o