I have had similar problems in the past. There is a maximum number of shared albums which is 200. Have you gone beyond that amount? Here is a link to the shared album limits. https://support.apple.com/en-us/HT202299
I have learned that a better way to create shared albums is by using the Public Website feature. In the Photos desktop app or on iOS, you can skip the invites which non-techy people often get stuck on. Select the public website option, and you will receive a link to your shared album that can be shared with anyone (even non-mac users). From this website, your family members can view and download the images. This is an iCloud feature that does not count against your storage so share away!