Out of tune when I capo the banjo

So call me a fool, call me ignorant...

But when I capo my banjo, it remains in tune with itself, but out of tune; the further down the neck , the more drastically so.

Is this supposed to happen? Am I tuning the banjo wrong, is it a problem with the capo?

It sounds fine playing solo, because as i said, it is still in tune with itself, but in any sort of jamming scenario, it sounds horrible!

All your help would be appreciated :)

Thanks!