I would expect there to be an administration if there was property to be disposed of - although I think it didn't always happen. I don't know about the Buckinghamshire Wills database, but the one held by Gloucestershire Archives includes both administrations and wills, without distinguishing between them in the index (and it contains Gloucestershire administrations from the early 1700s).
If you haven't found your ancestor's will in the online database, it might be that Bucks Archives keep admins separately - but I'm sure they will tell you if you ask.
Alternatively, if not all the deceased's property was held in the county, there may have been a will proved at at different court. Have you checked for a will held at Oxfordshire or Hertfordshire Archives, or a Prerogative Court of Canterbury will?