Charging slowly?

It seems that in most cases when I put my phone on the Nexus QI charger it goes into "charging slowly" mode. If I go back to it in an hour or two and look at the screen it will say something like "17 hours to full charge". I have to keep taking it off the charging pad and putting it back until it says only "charging". Never did this until Marshmallow. Why is it doing this?