Thanks for that, makes a bit more sense now, ie why none of the options worked.

Yeah, the connecting airports that I am looking for also come up on the normal A-B booking, but not necessarily with the 'right' times.

Let's say I was looking for LAX-PHL and wanted it to be via ORD. The normal A-B booking might give me two options for that, say Flight 1 for LAX-ORD and Flight 2 for ORD-PHL, as well as the second choice, Flight 3 for LAX-ORD and Flight 4 for ORD-PHL. Both choices are priced the same. What the normal booking function does not give me is a third option, namely Flight 3 for LAX-ORD and Flight 2 for ORD-PHL. Connections times etc are not an issue. But the normal booking function just does not give you all the possible choices. Multi-city does give you those choices, e.g. flights 3 and 2, at a similar price too, but then, just before payment, it comes back and says that those fares are not available.