I experience similar things with LAN award seats and OW -- the avialability of the airline FOR ITS OWN FFs is better than the inventory they grant to OW partners. Get on the LAN site, see every flight with award seats, then call EXP desk . .. no availability. My guess is you are facing a similar issue, that the airline grants only a subset of their award inventory to their nominal alllies.

Don't get me wrong, I have had many great award flights with all the OW partners. They do offer a generous enough pool to have satisfied my needs. However, I do see the differential inventory that you are experiencing and sometimes it makes a particular routing impossible altho the airlines own FFs could book it.