Ask HN: How do you get developers to review pull requests more frequently?

One problem that I have encountered frequently as a developer is people being reluctant to review pull requests with any regularity, often leaving pull requests to hang until their work is done, then dumping a lot of changes requested. Has anyone else encountered this problem, and how did they solve it? Currently I devote a lot of time reviewing pull requests to keep development moving faster through shorter feedback cycles, but as a lead developer, I wonder if this is an effective use of my time. I feel like I am depriving opportunities for my team to learn & grow through greater code exposure, but at the same time, I also understand that most developers are not great at context switching & reviewing code changes ends up being a context switch away from regular product development.