Simple thing which you may have tried already, but worth a shot:
Sounds like a very light short - possibly due to dirty contacts. Try cleaning the contact on your VMax 510 connection with a qtip and a drop of alcohol. Make sure you let it dry before reconnecting a
tank.
Using the Protank as an example (something I have):
Make sure your wick/atty combo in the Protank is secure. Remove the bottom cap from the Protank; check that the center post is secured in the ring; reattach the bottom cap. Attach the Protank securely to the VMax, but not too tight. Check your resistance on the device.
On or the other have both caused the same problem with mine. One other possibility is a blown atty or carto - which would either register wacky high or "low" on this device.
Hope one of them works for you. The VMax is a pretty solid device, but there is always a dud in any mass-produced equipment from time-time.