This may happen when:
1. You have a surface sich has to many vertices wich are not connected
Code:
+---+---+---+---+
| |
+---+---+---+---+
Solution: slice surface at those additional vertices, or remove them if possible
Code:
+---+---+---+---+
| | | | |
+---+---+---+---+
2. Vertice order is not right. Select surface and press ctrl+shift+o
(Or go to vertice menu and select "change vertice order")
Maybe you have to do this more than once.