MC's default is to cap the household at eight Sims. My current household is at six: my sim, his wife, and their two kids, plus the wife's sister and her son were moved in to allow growth in the sister's old household. No pets. I have a butler to help with three toddlers, but don't think he counts in the household since he's not controllable and is not in the portrait panel. I have the eight Sim cap turned off, but the TFB interaction is grayed out with the message that there is no room in the household. Since the household is at six (or maybe seven if the butler counts), I've gone and made another bald spot with all of the head-scratching. Can anyone offer any clues as to why this might be happening? Thanks.

Sure. See, when certain blood vessels in the scalp get irritated and begin to close up for various reasons and the sebacious glands begin to...oh wait, that's not really the part you were asking about, is it.

This is a very often misunderstood concept. MC only unlocks the ability to "overstuff" a household using MC commands such as Adopt, Instant Baby, Pollinate, and Add Sim. It has no bearing on Try for Baby, for that one the setting is in Woohooer.
NRaas > Woohooer > Try for Baby > Maximum Occupants for User Directed

Zero would mean no limit, but is it possible you have this value set for something else less than 8? And I'm not really sure if the Butler counts. NPC Roommates are not controllable either though I think they do count, but the Butler is actually part of the service household.

Oh, and if you don't even have Woohooer then you cannot adjust the limit. But then it doesn't sound like your household is completely full, either. No plumbots around? And there is no chance one of the other sims (presumably the sister-in-law) isn't already pregnant?

Thanks, igazor. This is why it's puzzling. I have Woohooer installed, but didn't mess with that setting, so it's still at the default (eight, I assume). No Plumbots, no roomies (except the butler), and no one is pregnant. I'm not able to get in-game at the moment, but will check at the next opportunity to see if I changed the Maximum Occupants in Woohooer and simply forgot.

The default setting for Woohooer is 0, which means there should be no such cap if it hadn't been changed. But another thing to try is to break the link between Woohooer and SP, if you have that set, and see if that unlocks things.
NRaas > Woohooer > General > Link to Story Progression > False

It could be that you have a household cap set in SP that Woohooer is trying to honor. Or it thinks you do. The link gets things a bit confused at times.

You were correct. The household cap was set to disable TFB at 5 (twins pushed me past it). I had delinked SP and Woohooer a while back after I came across reports of weird problems with teen woohooing and apparently capped my household at 5 at about the same time to prevent the fallout from all of the risky woohooing that was driving me nuts at the time. Good call and thanks for the help.