iamfedora There is much more than soulmate status to a marriage being accepted/rejected now. You need to make sure they have a high romantic relationship, are gender preference and sexual orientation compatible as well are mutually attracted to each other. Likes/dislikes and turn ons/offs come into too as you need to set at least some of those so they are mutual, otherwise the game will give them random ones that may be mutually exclusive. So, my advice is to take the 2 sims to CAS and make them more compatible by looking at all those things and make enough changes to where they are more compatible when you look at their attraction using the relationships options.
Hope this helps.