truncated view -> OpenCascade 5.0

I have created an application, but there is a slight problem with the viewer i.e. the objects displayed such as a line is partly truncated/cut-off. Why am i having this problem and how can i rectify this problem?

my source...no clipping
V3d_TypeOfZclipping oType = V3d_OFF;
_V3dView->SetZClippingType(oType);
_V3dView->SetZCueingOff();