Chronographing .177 air rifles

I recently puchased a new Gamo air rifle and wanted to chronograph speeds of different pettets I purchased for the gun.  I set it up last night but my chrono won't pick up the shots.  Is there a secret to chronographing .177 air rifles?  I have used my chrono for arrows, shotguns, and rifles so I know it works.

Any suggestions would be appreciated.

Alan

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I can't give specifics, but assuming you have a good battery in it and there is some acxtivity on the led screen, it is usually lighting.