This happens to me constantly on my motorcycle charger. I can drive around for a while using the GPS, almost everything indicates it is charging the battery, but it really is discharging. I would replace the cable and that would correct it for a month, then it would start doing it again.
The only way I can tell it isn't charging is to use the 'Battery Monitor Widget'.
What I have learned is that I can take a small pair of pliers and gently squeeze the connecter to tighten it up and it is good for another month or so. But I need to keep an eye on it. None of my house cables do this, and I have tried three different brands of cables on the motorcycle. Must be the vibration or something. Very odd.