I believe this is the best answer:
"I'm sorry to hear you've recently not experienced the product at its best. Right now we're rolling out a new algorithm in some of our courses and it’s exposed some shortcomings which we’re already in the process of fixing. Essentially, the algorithm should adapt to your personal memory strength quicker than it currently does. This means correctly answered words come back a lot sooner than before, resulting in not seeing new words for a long time. Since we’re rolling out the new algorithm course by course, it’s not been applied to Russian or Spanish, which explains the differences you’ve noticed.
This issue can be more frustrating for those with better memorisation abilities. If you find that’s the case for you, until the issue is fixed I’d recommend checking out our new challenges area which is full of speaking, reading and listening exercises."