Thank you for your reponse! However, even after following your suggestions, the custom tag is still missing. I sent a message today asking if two sessions of TMG could be open at the same time so that I might do some side-by-side editing.
Quite a while ago, I created a tag, OBIT, which has worked fine for years. Recently, however, it has disappeared from all instances, and I cannot create any new ones. Does anyone have any ideas on how to remedy this? Thank you.