gluTessXXXX problem

Hi, I draw polygons in a openGL scene with this functions.
GLUtesselator * pTess = gluNewTess();
gluTessNormal(…);
gluTessCallback(…, glBegin);
gluTessCallback(…, glVertex3dv);
gluTessCallback(…, glEdgeFlag);
gluTessCallback(…, glEnd);

gluTessBeginPolygon(pTess, NULL);
for ()
gluTessVertex(pTess, pPts+j, pPts+j);
gluTessEndPolygon(pTess);
gluDeleteTess(pTess);

the fill-polygon are drawn ok. But if there are some layers with fill-polygons which overlaps (but equal z-coordinate), the new polygons are not drawn fully above the old polygons. The scene shows some areas where the lower polygons are draw above the last polygons.

why???
How do I resolve this error ???
thank you very much