What about rude sitters who agree to things then are a no-show or you don't hear from again?

As Karen mentioned you should contact Rover support @ [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team]

You can't leave reviews. However, you can convey your dissatisfaction to Rover Customer Support. You will need to contact them. This isn't the place.

Did you call/text them? Since you were expecting the first one to show up, did you call or text when she didn't show up on time? If you did, but she didn't answer, that at least would tell you she's not handling her business right and you could tell Rover, but I don't think there are any rules set that they are required to show up. (No, not taking their side, just stating that we do not live in a perfect world and it happens).

The second one, she maybe thought you were not a good match but didn't want to tell you, or something happened, got a booking request from someone else etc. Did you call her to follow up with her?

If they really just flaked, and you did call them, then it only shows they are flaky and you do not want these people working for you anyway. I know, most of us would cancel if unable to show up but not everyone is like that.