I don't think it's quite fair to say Kumbuka is "showing no interest in the females". He's very friendly towards them (well, apart from Zaire who keeps to herself) and I've seen him exchanging grooming with Mjukuu, but perhaps in a more brotherly way... still, early days!
mmmm... the thing is, however they behave initially is usually how they will continue to behave longerterm. A strange 'normal' male would usually make his presence felt at the start with energetic display, chestbeating and generally putting the females in their place a bit. Once he's got their respect, he calms right down and things click into place. It doesn't sound as if that is happening here.
An alternative option is one of the females will encourage mating through initial bouts of play- this sometimes happens when a male doesn't have the 'dominance' factor in the first place.