also to see if you have duplicate edges or not joined go to custom polygon display and click on the border edges button, set the edge width to what ever is good for you, I have mine set at five (you may need to click on display/polygon components/border edges too, but I'm not sure). i am not at home to see if last one needs to be done but edges will be in bold now and you can see them ..if you are at an edge you can tell if you have two close together then that is wrong ...

also if you do right click mouse and do vertices faces ...if you see two faces on each other or two lines then you have more than one face stopping you from converting ..if you cant figure it out just post up file so someone c an look at it and try to figure out where it is messing up