It's a confusing system, one I can't completely explain and I'm on a different machine and don't have my saved links. You have several things you have to balance out in order to vote and participate. That is your personal "Voting Power" and Your "Bandwidth." If your Bandwidth goes zero, you can't do anything, nada, zip. If your voting power goes zero, your votes become worthless. Every time you vote, you use a percentage of your voting power. Now this voting power replenishes about 20% per day (takes 5 days to completely restore). It all depends on how much Steem/Steem Power you have but I believe you can only vote about 10 times a day without depleting your Voting Power. This is done I think to help focus voting on worth wild post and not so much people just voting up friends etc.

You can view both Voting Power and Bandwidth on Steemd.com (I believe you insert @/your user name) or you can view Voting Power on Steemdb.com (can't recall how to find accounts. I personally keep them both open and monitor them regularly. Now there is a new Google Chrome Steem header or what ever it's called that will display a lot of Steem info like a ticker symbol above or near your page tags. I haven't played with it yet and can't remember if this includes voting power and bandwidth.