Just got an error message when I attempted to upvote a post. Was pretty surprised, I didn't realize that this could happen.
I have discovered that, more or less, I used up too much steem power upvoting people and my content was not generating enough upvotes to compensate for it. Its all about the steem power.
Sounds like 25-50 upvotes per day for us n00bs until we gain enough reputation to muster more...
Here is what the message looks like:
This post from 
https://steemit.com/steem/@arhag/where-does-the-money-come-from-a-look-into-the-economics-of-steem
Essentially, SteemIt is constantly taxing us to provide incentive to miners to operate the blockchain. (Steem has two types of wealth taxes - hidden as an inflation tax). It taxes liquid STEEM at approximately 50% per year, and it taxes Steem Power at a variable rate of less than 5% per year) If you're not adding steem to your account, either by generating through content creation and curation OR buying more steem power, eventually you'll run out of steem to participate. To add a wrinkle, if the market cap for Steem increases at a rate of aprox 5% or greater, users don't need to worry because the gain will take care of their taxes allowing them to participate.
He also walks through some of the psychological narratives driving this platform. Quite the interesting read, highly recommend it. First great source of information.
Ultimately, be careful not to over upvote if unlikely to be reciprocated.
Also, anyone realizing any best practices to avoid this beyond purchasing Steem?