Hi,
I was just about to post, that I figured it out.
Actually, I use Livecycle Designer in SAP (transaction SFP). There seems to be a bug (one of hundrets by the way).
If I click on a field, that does not have text-property e.g. subform, text-option menu greys out.
Afterwards it stay this way until I click/doubleclick into a simple text (not text field). From that moment on, text/font-option menu is available again, until I click on something without text. => So pay attention and click wisely
coming soon: "SAP meets Livecycle Designer - a (Bug)Lovestory", hardcover 124 Pages. ; )
by the way: SAP does not sopport interactive forms higher than Reader 8.1. Its statement "8.1 is fine enough for everything you need in a SAP/Adobe Interactive Form". So beware of using XFA higher than in 8.1. sad, but true ...