Upcoming Hard Fork Will Fix The Miner Witness Queue Issue
It has probably not escaped anyone interested in Steem mining that the miner witness queue has been completely dominated by one user recently.
A lot of people speculated that it was due to GPU mining but it was later revealed that it was possible to game the queue using some clever hacks. Since only 1 of 21 blocks are produced by miner witnesses the blockchain can't be compromised using this exploit but it does make it almost impossible for other miners to submit a POW.
It didn't take long for the core devs to respond with a hard fork which should fix the issue: https://github.com/steemit/steem/issues/256