These are probably orphaned blocks which are no longer valid, so cannot be exchanged to bitcoins. I am clearing this stuff in batches. Most of old stuck blocks which were valid were already exchanged and paid (there were more than 10 BTC stuck pool-wide which were exchanged and paid out in the last two months, what's left is most likely orphaned). More details here: https://bitcointalk.org/index.php?topic=448649.msg8138087#msg8138087