The detent is under the cover on the opposite end from the handle. Just take the two screws out and look at it. There are several types of detent setups. They all use springs and balls. They get sticky/rusty and need cleaned up usually. Sometimes a spring will break but not usually.
Here is a link to the current valve Energy Sells. It look a little different than your valve but not by much. There is a parts break down right on the page. There are also instructions on replacing the detent part of the valve.