Not even Aeroplan always knows so it is best to do as much homework yourself before calling them.
For example, I couldn't book Calgary-Denver-Lincoln, Nebraska on Aeroplan.com even though Calgary-Denver showed no problem and Denver-Lincoln showed no problem as two separate trips on same dates on Aeroplan.com.
Initially Aeroplan didn't see it either but once we got started and I explained things, the agent was able to get my flights on one round trip reward ticket.
Others have noted issues with LX rewards (I had similar issues).
I don't know why there are so many IT problems with ac and Aeroplan but it is getting better. Let's hope it will be purr-fect one day