Tub Won't Attach

[dsw98]

I'm trying to put a tub on an exterior wall but it won't let me. However it will add it to the interiro wall(see attached), any reason why? I'm pretty sure both my walls share the same type of structure, I'm lost here.

[Steve_Stafford]

The wall that won't accept the tub has a 2'-0" offset from the base level so Revit can't "find" a wall to attach to.