The thing that gets most people is the capacitor on VUSB.
Should be at least .22uF (bigger works too, it's just a DC filter)

Second most popular failure is with the oscillator configuration.
The PLL structure in those chips is fairly complicated, and it has to be right to work.
What crystal are you using? and, what do your configs look like?

But first, your explanation is a little confusing.
Which type of communications are you trying to achieve? HID, CDC, or mouse.