The problem is usually caused by Carrara not fully understanding what is the "inside" and the "outside" of the object. Sometimes you can correct it by deselecting everything and then just selecting one triangle and filling that one. Then Carrara will fill the others when you select all and use the "full polygons" command. I don't see any problem with the construction geometry in your image, but occasionally you will find that a vertex is wrongly connected if you dolly around the figure. Once that is fixed, all of the polys will properly fill. If neither of the above things help, select the whole object and go to the Weld command in the top menu and use the "Custom Tolerance" method setting an appropriate distance. The goal is to weld very close vertices together because sometimes there are overlapping vertices which LOOK connected but aren't.