Photoshop Mask Mode problem

[mohityadavx]

Hi!

I am using photoshop CS5 and I am suffering from a strange problem, photoshop is not selecting parts of photo in mask mode . for ex:- If i am selecting a face it will not select some parts near the eye, leave some more parts near the eyes , or some space at cheek. This makes the whole exercise futile as i if i change skin tone then it appears very weird like skin disease of superior whiteness at one place and dark colour at some part. Please help me out

It is not selecting completely even those images i have worked with earlier without any hassle. I am a newbie so it may be some small problem only something with setting i guess.

bump........