Just a couple of suggestions. Do you have the metal contact cap on? The one that fits over the spare battery compartment / lamp assembly? Check that it is on snug. Give the whole light a good cleaning with alcohol and q-tips. Clean all the contact points. Other than that checking it with a multi-meter would be the next step. Good luck
Just because you test the batteries and they show as "good" doesn't mean they'll power the light. Most battery testers do not put a significant load on the battery, esp. a load equiv. to a scorpion lamp. try a new set of batteries...
Similar to what dano points out, in my line of work I often see fuses do the same thing. The fuses (rated for 250 amps) will light a test light but they will not pull the necessary current.