Thanks for all the input. We actually showed the agent that we could buy the ticket and she said she still couldn't see it as an option for rebooking although she could see that revenue seats were available. I think that's a point that we will emphasize. If seats were available for sale the agent should have been able to book us on that flight. If she couldn't, that's a service failure IMO.
The app issues that I was referring to were likely connectivity issues because the app kept timing out. I just mentioned that to explain why we didn't buy the ticket while we were still at PVD. While my friend was futzing around with buying the ticket we were already in the car and on our way to BOS.
The cancellation was for WX.