Chester seem hesitant to breed from him, or there are direction coming from elsewhere. Seems strange to me that Stuma was cycling regularly with Mbuti there, they brought a new male in and BANG she gone awol.
I believe M'buti is/was interested in trying to mate Stuma but isn't regarded as being old enough to cover her properly yet. He was certainly very interested in her when I saw them in adjacent enclosures a couple of years back now. How old is he now- about six or seven? Possibly the arrival of the Marwell male disturbed her somehow and disrupted her cycle. That male did not stay very long either so I don't think they gave them a decent chance of success. I still think M'buti will succeed with her in time.
He still is very interested. The male from Marwell was only ever there for 6 months not enough time with Okapi who need a very long introduction. Bongorob answered the question a PM. I'll send you one in a bit.