The Slow ring on the Windows Insider Program has always been a strange middle ground, with the Fast ring on the cutting edge and the Release Preview ring getting cumulative updates that are slated for public release. Windows Insider chief Dona Sarkar once told me that the Fast ring is meant to offer weekly builds, while the Slow ring should get new builds monthly, but that's not always the case.
In fact, you can see for yourself with Flight Hub. The issue was particularly common with Redstone 4, where the Slow ring saw no new builds between 17025 and 17074.1002, even though the Fast ring had received eight new builds.
Since it was an issue with the last update cycle, it should come as no surprise that Microsoft is now coming up with a solution to make sure that things like this won't happen anymore. Moving forward with Redstone 5, the company will be servicing Slow ring builds.
This means that there will be more frequent builds in the Slow ring. When a build is released, it will be designated for the Fast and Slow rings, being offered to the Fast ring first. If there are no new issues in the feedback, then things will work like they do now where the build is released to the Slow ring. If there are issues, the fixes will be applied, and then it will be released to the Slow ring, so you might end up seeing something like 176xx.1 in Fast, and then 176xx.5 in the Slow ring.
Ultimately, the Windows Insider team told me that it aims for beta quality software in the Slow ring, with alpha quality in the Fast ring, similar to what you see in the Xbox Insider Program. The bar will still be set much higher for the Slow ring than it is for the Fast ring, but you'll start seeing more builds.