Default [Q] Nexus 5 "Ok Google" issue.

I bought Nexus 5 before 2 days ago. When i started Cellphone first time and tried "Ok google" it was working fine.

Now after 1 day i did upgrade to 4.4.3 and 4.4.4. Now "Ok google" word detection seems not working for me. I tried to restart phone 2-3 times but didn't worked.

Some times it works after 4-5 speech attempts and for that i have to keep my cellphone near my mouth.

So i want to know how to debug the issue, like whether it is issue from hardware (i.e mic) or issue from OS ?