Admob bannner ads not taking full width

Change

ads:adSize="BANNER"

to

ads:adSize="SMART_BANNER"

Smart Banners are new ad units (as of v6.0.0) that will render screen-wide banner ads on any screen size across different devices in either orientation. Smart Banners help deal with increasing screen fragmentation across different devices by "smartly" detecting the width of the phone in its current orientation, and making the ad view that size.

Read more here

you can use FULL_WIDTH

example ads:adSize="FULL_WIDTHx50"