Ok, instead of doing the stuff I'd planned to do for the last 2-3 hours I've ended up investigating this slow blocks issue and have come to some conclusions:

First of all the problem is that there aren't enough coins staking over the last 24hrs - prior to that it was ok so I assume there are sufficient coins in circulation, its just a lot of them have either staked or been moved, so they are stuck in "limbo" currently. Until a few more blocks are confirmed (txes need 10 blocks to confirm) they will remain in limbo.

The problem is made worse because SPARK has a hard coded minimum PoS difficulty that is relatively high, making it harder for stakes with a low coin age (number of coins * their age) to hit an acceptable block. This can be adjusted but it will require a hard fork.

I suspect this problem is linked to Bittrex re-enabling their wallet so a lot of coins which were staking are now being moved around, if that is the case the problem should fix itself as blocks are slowly found and more txes confirm so the coins start staking again.

Unfortunately neither Ed or I have many SPARK which doesnt help, of course donations are most helpful at motivating us to do more work - even if they will take a while to arrive currently feel free to send to Sg2dujC3jMa9j1ff41sjAZRV4mZkjwgN6G - thanks!