I got this from support:
"We have upgraded our algorithm at the end of December and for some learners with previously low accuracy rates it resulted in greater repetitions of the same words. This change has been rolled back as of this morning and hence the speed with which your repeat words are going to re-enter the queue is back to previous. At the moment you need to complete whatever the outstanding repeat stack there is for you, after which you should see a lot more new words."
It seems to have worked.