Sioux Drill Motor and Keyless Chuck

Anybody know how to get the keyless chuck to fit tight onto the drill motor? The drill motor shaft just turns as the chuck is snugged up and then quickly disembarks if the drill is aired up and trigger is squeezed. I can't figure out how to hold the drill motor fixed to tighten the chuck. I asked Sioux and Brown Tools, but no response, yet.
S

Called brown and they explained it. Chuck up a drill bit and drill some holes. Chuck gets tight. Duh.