D90 and Nikon 35mm f1.8G focusing issue

[monsters]

Hi everyone,

Recently I have bought a nikon 35mm f1.8G lens and I realise that a few of my photo is out of focus. My lens was focus on the subject's face but when the picture came out the subject's face is out-of-focus and you can see that the background is clear.

Is my lens having a focusing issue or there's something wrong my technique/setting or camera.

I am still new to photography. I hope to get some advices from you all

Thanks for your input.

p.s. i will try to post a few photo up as example. By the way, 1st photo , was taken at f2.2, iso250, shutter speed 1/250.
And second wedding image is taken at f2.8, iso200 and at 1/125.
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