Coindesk profiles some of the lingering technical issues with the Ethereum network, including a glitch that resulted in unnecessarily high transaction costs as well as a proof-of-work test network that is splitting into incompatible forks.
Adding to this, developers have been struggling to get network nodes to sync to the blockchain. Those that try either fail, or are faced with a long wait. Stepping back, any new node that wishes to mine ether or run the blockchain as part of its own wallet is faced with this problem. And while the issue has circulating for some time, it has worsened in recent weeks. According to Peter Pratscher, who runs the mining pool Bitfly, it's down to a failure of a function called "warp sync" as a result of the current size of the ethereum state – a figure that equates to a current sum of all of ethereum's computation, and is stored by all nodes.