Yes Jonny B you are filling the lighter wrong. I tossed many lighters for years because I didnt know what I was doing wrong...
Before filling the ligher you have to turn the "intensity" or "flame setting thingy" all the way off so that it closes the release. IIRC the pressure of the butane being pumped into it causes it to freeze the release tube and if not turned completely off with cause a leak. After you have done this even 1 time it will cause the lighter not to function properly.
On "most" butane lighters there is a +/- sign at the bottom, just turn it all the way off, flip the lighter over and fill it in quick 2 second burst's. I have never had a problem after I was taught that.