Well, I'm not sure why the mod did that, but it sounds like you've isolated it to the computer, which really don't use a computer for charging as if anything goes wrong, you have the possibility of damaging your computer.
Really, it's best practice to buy an external charger and charge that way. Sometimes batteries will charge unevenly via USB for one, and also it's possible that over time your USB port MAY wear out, and it's really more for updates. You can certainly charge it that way in a pinch, but you would be better off with an external charger (you can get a 2 bay charger fairly inexpensively, I use a Nitcore 2bay intellecharger) as well as another set of batteries so that you can just switch them out.
Less wear and tear on the mod, less strain on the batteries, and best of all not having to wait lengthy periods of time for charging, you can swap out batteries and go.
I don't know if your mod is defective or not, for having this issue, I'm afraid I don't own one. But if it's functioning fine with a wall-wart, that sort of sounds like it may be a quirk related to computer charging-- maybe not getting to full charge? IDK.
Anna-
