Segment by segment is the only reliable way to check on the phone.

If several calls results in the same agent answer of not available, I would conclude that the award is not bookable at this time with Avios - either by design or because some sort of system limitation/lag/glitch. So far for me I have been able to access the same saver AS flights with Avios as with AA/AS miles by calling back when I got the wrong answer but from reports here it seems this is not always possible....don't know why.