Can't Add Audio Captions for Clips Under 1 Second Long

I'm trying to add the closed captions for several audio clips that are very short. These are part of a conversation between two people on the phone and some clips are just one word long.

The clips don't have a caption bubble already created when I go into the Closed Caption Editor. When I click on Insert Caption, the Insert Caption icon turns gray and nothing else happens. Please advise how captions can be added. THANKS!