First, have you tried it with both a 9V adapter and a battery? Sometimes, the battery connector might be bad, but it will still work with an adapter or vice versa.

If it works with a battery but not an adapter, make sure you’re using the proper voltage and polarity for the adapter. The vast majority of EHX pedals use a 9V NEGATIVE TIP (the negative tip part is very important) adapter. The official EHX adapters say 9.6V, but it’s the same thing. A FRESH alkaline battery produces about 9.6V. Also, the adapter must be making good contact with the tip, that is the center part. Sometimes an adapter will fit in the hole, but it will not be making good contact with the tip because the hole on the inside of the adapter tip is too big. If you’re sure you’re using a good adapter with the proper polarity (negative tip) and voltage, but it’s still not working, then you probably have a problem with the adapter port. They can get burned out if someone uses the wrong adapter – too much voltage, wrong polarity.

If it works with the adapter but not a battery, make sure that the battery is good. Test it with a battery tester. EHX has been shipping their pedals with carbon zinc batteries which can change the sound of some types of distortions IN A GOOD WAY, but they don’t last as long and they can discharge without even being turned on. Test it with a FRESH batter that’s producing enough voltage to make the pedal work. The truth is that most pedals can work with a voltage anywhere from 9.6V (fresh alkaline) down to 7V (discharged battery) or sometimes even lower, but some pedals will simply stop working below a certain threshold so make sure you’re using a pedal for the first time with a fresh batter.

If you’re absolutely sure about the adapter and the battery, then the only other thing I can think of is that either you’re plugging your cables in reverse – Guitar should go into the right and amp out from the left, or possibly one of your cables is bad, or possibly one of the jacks is loose so that it works intermittently.

I guess there is one more possiblity – your vendor is lying to you about their testing.