Droid MAXX will not charge from portable battery

One of the reasons I got the VW Droid MAXX last year was it had a battery twice the size of most phones at the time.  But there were times when I still needed a quick charge when I don't have access to wall outlets.  I had several portable batteries that worked with my prior Droid 4.  But they don't work with the Droid MAXX at all.  When connected, some show the phone is charging for a few seconds, then it changes to not charging.  Others never even show the phone charging when connected.  Any thoughts/ideas why?

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