Hey! This is a super-duper game, and I love how involved the developers are in the community!
I've hit a similar snag to the OP. I was also playing Friendly (version 0.0.254), and I have the same issue. I'm on day 75 or so. Trade ships come pretty frequently, but it's a random mix of industrial and farming vessels - I haven't seen a slaver since the first one a few days in. Because it's an easier level, the raids are infrequent (every 10 days or so) and usually drop damaged raiders; however, I don't seem to have the option to capture them regardless of their health.
Let me know if I can provide more useful information; I'd really like to expand my prison. Err, my colony. I meant colony.