I have had the same problem in the Hebrew course, and started using TinyCards (Duolingo’s companion ap) to review vocab. It’s made a world of difference. You don’t get exp or lingots for Tinycards, but it makes the Duo lessons and reviews go a lot more smoothly.
It means to strengthen your skills fully. Over time, these skills deteriorate in strength as you go over them less. It's always good to review skills occasionally, and when you feel ready to keep going, keep going.