My Blue collection is the same way. I'm practically complete through 1923, and then I drop off to no more than 50% for years between 1923-1940 for regular issues/commems, and less than that for BOB.
Ever so often, I decide to monitor
ebay for Russia collections, but the few that have good coverage of 1920-1940 have brought more money than I am prepared to spend.
I do think this is one of the fascinating aspects of using the Scott Blue International. Since most of the stamps in the album don't have that high a catalog value, as you start to complete the album, you often find cases where low catalog value is inversely proportional to availability.