I cannot for the life of me successfully romance Dallas!(I haven't tried any of the other guys yet)
This is a common problem that I seek to fix in the next patch. The problem is you are probably playing the game too well, so the objectives complete faster, so the game ends quicker - and the romance scenes just don't trigger fast enough. So in the next patch, before the game ends, it will check to see if you've completed the romance and ask you before proceeding.