if you are plat or 1k with free award changes, try just booking any day that will ticket. Then after it tickets go in and change it for the flight you want and know was available on an individual search.

I booked a weird routing this way. One of the segments wouldn't show up in the multi-destination search, that I was able to see as a one-way. So I booked it with a date that worked then changed to to the one I wanted. (There was another one I tried this with, and it didnt work)

this might work, and if it doesn't you have free re-deposit as a plat/1k. If you do cancel, the flights you had booked might not be available right away after cancelling.