I had a similar situation. It can be done, but is quite difficult! It helps if they have a dick, because you can then stroke it while groping them. To do this recently, I found that I had to: hypnotize her; give her a couple heat pills; lube her; give her a breeder pill, just 'cause; ride her until she was close to orgasm, switch back to groping, stroke her cock; and just...got lucky, eventually, that she finally came, lol. It also helps if you don't have the overstimulation system enabled, as that can make it even more difficult. I think I had it enabled, still, though. If the slave doesn't have a cock, though, I don't know if it's possible - I think that was the only way I could push her up and over the edge, after a point. Good luck!
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-- to add to this, obviously worth it to keep in mind their other preferences, in case some things you do hamper or help them reach orgasm. c;
If they don't already have a cock, you could temporarily give them a peen by giving them a dick pill. Do the deed, and then give them a MorphAway to remove the member, if desired!