It's starting to really piss me off now. I've been trying for almost 2 weeks now. Whenever I want to use Game Sync it tells me that I've run out of energy. But it's never working for me. I've never changed the time on it. So why won't it work?
Changing the DS clock or using the game in another DS will result in needing to wait 24 hours before a Pokémon can be put to sleep again. - Quote from Bulbapedia
Did you ever switch the DS systems you are using? If so then stick with your main one. Leave the game in there for one whole day. If those aren't the cases you might wanna look on the Support for help. (Maintenance period and closure of the DW for B/W and B/W 2 is coming soon)
