joining tris with j key

I’ve made a human head and tried to join some tris with the JKey but it doesnt work. In the same scene i put a plane ( as a test), ctrlT to make tris and J to join them (make quads again) and that works fine. Why cant i make quads with my human head model?

Thanx, Twan

Its because the angles on some of the quad are two extreme. In times when J does not work, just delete both tris (x->Faces (with both faces selected)), and then rebuild the faces with F. Should work.

To show an example break the plane into tris, then move two diagonally opposing corners down one unit, and try to rejoin (should not happen).

I don’t know why this happens, perhaps a coder can come along and explain it.

Thanx a lot for your quick reply. I tried your option, rebuilding with the F key, but i couldnt stand the fact that it didnt work the way it should. I think indeed that the angles are extreme.

Thanx, i go for deleting and rebuidling.
Twan