I think the original band/combo was in pieces by 1940 and that would explain the difference. The original members were great together. Pulling two or three out made all the difference.
To be fair, some also say that the early years background music all sounds the same. I tell them listen to Mama Don't Allow It and tell me that.
I still think the later Ink Spots stuff is great, just not as exciting to me as their early jive. And yes, their Mama Don't Allow is my favorite version of that song.