The Oneworld booking tool is extremely buggy and regularly does not return what you want. However by the "(max 34000) miles" I suspect it is trying to book an XGLOB34 rather than a XONE3. You may be able to specify Oneworld Explorer under the Fare Eligibility tab top right. Otherwise clear your cache and try again. Alternatively if you are happy to fly the LHR-LAS sector under the AA Codeshare number the AA RTW Desk might well be your friend here.