One thing I have seen throughout, is that not all SIMS can become Soulmates. The majority of them can gain that status, but not all. Amazingly enough the SIMS I thought could become soulmates never could gain that status, but was amazingly surprised when I've seen many others gain the status. It does not take full bars across to obtain the status....maybe about halfway on both.
I have never figured out the personality traits that can allow this to happen.....but to be honest I've never tried.
To complete the quest, I find SIMS that become soulmates first, then make them my best friend, and marry them. I never start with the BFF first.
-AL2