Occasionally hotels are sold out but they don't close down the reward rate, only cash rates.

(I have no idea why they do this).

I don't think it's a question of the rep not being competent, but rather there is only availability if you use points. As such, if the Free Weekend Night books into cash availability, you can't use it - unless, I guess, you can use Diamond Force and the certificate is compatible with that.

I'd also add that if you're booking >9 months out, it's more likely that the hotel simply hasn't loaded any cash rates, rather than that they're sold out, in which case I would just wait.