Hi
I have tried on all the options given below. Firstly, the face (its actually a protective shield for the helmet ) was created using the append to polygon tool.

I get a feeling there is something wrong with this as I am not able to select any point on this surface with the split polygon tool. I have created a polygon cube and used the split tool with the current settings and its working fine.

Definitely, there is a problem with the face. I deleted and did it once again, with no luck. There must be some step I am missing here....

gp.