There could be a few reasons for the chronograph not reading shots. The first thing we suggest trying, is to replace the battery with a fully charged, recently purchased 9V battery. When these batteries begin to lose their charge, even sitting in the drawer, it can affect the chronograph’s accuracy.
If the battery is good, it could be the cables to the sensors have began to come apart. You might find a sweet spot moving them around near the ends. In that case, replacing the sensors would be the best bet.
Finally, if the issue is not the battery or cables, then it could be something related to the internal circuit board in the chronograph. In that case if it’s eligible for repair, you could send it in.