Does it show up immediately or later?
If Later the dull spot shows up, it probably means that you sanded through the finish and with a wax finish, you put the shine on and it wore off. Spots is indicative of off center (some call it out of round, which is technically not correct.) You are wearing through in high spots. That comes from too much sanding pressure, slightly bent mandrel, dented point on the tail stock, and possibly others.
If immediately, the fellows above gave the majority of those causes. However it can be also "sanding through on high spots" caused by the things I mentioned in the above paragraph.