I have the same problem. Could be that your usb connector is coming loose. Try gently wiggling it side to side and see if there is any play. The usb port on the n900 is shody by design and can easily come loose or even break off merely as a consequnce of what would be considered normal, every day use (I didn't abuse mine, yet it started coming loose a mere three months or so after i bought the phone). At this point, plugging in my (or a friend's) n900 charger goes completely undetected. strangely, the charger from a $40 Samsung phone works perfectly. Also, charging via usb cable worked well until recently.