Items will not go into the hopper if it's powered, even if they have space.
(On an unrelated note, you can make a funny item-trampoline by placing a wooden pressure plate on top of a hopper and then throwing items on the plate.)
Comments 3
"Powering the hopper now also stops it from taking in items (see comparison between 13w02a and 13w02b[3])."
Source: http://www.minecraftwiki.net/wiki/Version_history/Development_versions#13w02b
Duplicate of MC-7178, please use the search function to see if your bug has already been submitted. Currently over 42% of tickets are being closed as duplicate.
I believe that is intended. The normal state is to pass items on, then halt when powered.