Very strange. I was trying to book a RT from SFO to YYZ online. If I enter in my dates as a round-trip it tell me no seats available.

However, if I try to book as two one-ways - same destinations, same dates - I get available flights in both directions.

I suppose the downside of booking a round-trip as two one-ways is you have to pay two cancellation penalties if you decide not to go.