I had this problem intermittently.
For example, I found that if I removed a Supporting Detail from some of the Medical Expenses Step-by-step forms, I could no longer add one back in, either by double-clicking, typing Control+I, right-clicking (which never seems to work any more), or using the Edit/Add Supporting Details menu.
However, by exiting TurboTax (saving my work) and restarting it, I was able to add one again.
I highly recommend that anyone encountering this issue restart TurboTax and that will likely resolve the issue for you.
