Use a Q-tip dipped in alcohol and gently clean the contact that connects to the positive end of the battery. Also, clean both the top and the bottom of your batteries. Dust, debris,
juice gunk, oxidation, what have you might be causing the battery to break contact.
One thing that might be going on too is that the indentations created by the Vamo and other mods on the two ends of the battery may be causing them to lose contact.
What kind of batteries are you using? Does it happen with all of the batteries you've tried? If they're the grey DKI batteries (come with some Vamo kits), and they're being used regularly, try getting some AW, Panasonic or Efest batteries. Those DKI's are junk and if their chemistry is getting screwy, the protection circuitry in the Vamo is just doing its job.