Like you, I lose Franklin & Sophronia following their marriage.I thought that perhaps Franklin took her back to the Dakota Territory, but couldn't find them there.Actually, it would appear that the Bartlett's left SD, after a brief stay, and returned to Wisconsin & Minnesota.
I then looked for Franklin & Sophronia in Washington state - Soph's brother, John Henry Putney moved out there.Again, no dice.
I also checked Buffalo Co., WI marriages for Sophronia Bartlett and a later marriage for Franklin, thinking one of them may have been widowed.nope.
I have done a search on every Fran* Bar* & Soph* Bar* in 1900.Nothing matches to our folks.
I believe this is what is called a brick wall.Got a chisel?