Having problems welding

Hi all,

I'm having a bit of problems figuring out why certain parts weld and others don't?!

I can tell you that the 'running man' is a single outline whereas the swimmer and bike rider I created with different lines using the freehand tool. I've tried to weld, combine and group each image but to no avail. I'm just not quite sure what makes them different to the text and running man. 

Any help would be HUGELY appreciated as this has bugged me for over a year now and I'm finally going to learn what's up!