When to cancel an unconfirmed booking (second request)?

I agree with Mary. She may not understand that she needs to confirm the booking through the site. She may think that verbally confirming with you is essentially the same. I usually send a message like:

First time it expires...

"Hi (client), I'm glad that our meet and greet went well and am looking forward to my stay with (pet's name). Could you do me a favor and log into the website to click "Accept" on the booking request? Rover's insurance doesn't cover your pet until the booking is confirmed by both parties, and I want to make sure that _ is covered in case of emergency. Let me know if you have any issues navigating the site/app."

If they still don't confirm the booking...

"Hi (client). I just wanted to check in again about our booking. I noticed that Rover hasn't received your confirmation yet. Please let me know if you'd still like to book these dates. I'm still really looking forward to working with you and (pet), but I have a couple of other clients that are interested in these dates. I know things get busy, so I'll wait (X number of hours) before booking another client. Please reach out if you have any questions or need assistance!"

This makes it very clear that you are understanding of any technical issues or barriers preventing them from booking the dates, but that you are also setting a deadline for a response so that your business remains profitable.

Shane's answer is spot on. I'd like to add one more anecdote from my own experience:

After communicating several times with a client who seemed to not understand what their next step needed to be, I felt weird emailing YET AGAIN. I contacted Rover support and explained the situation, and someone from Rover support entered our online conversation and explained step by step.

It was very helpful, and worked like a charm.

Best of luck!

If the M&G with the two adorable-sounding pups goes well and that client proceeds with the booking, go with it. The first client that hasn't committed or paid is potentially showing you already how the stay might go: expect late dropoff, no responses and all the other inconsiderate things that sitters may be subjected to. You've followed-up with the first one enough times and apparently are in demand. Go with the one that treats you with respect.

HI Camille: I'd contact the original client (with the expired stay) and tell her that you have 2 other clients that want to book with you, so she has X amount of time to confirm booking. If the time passes and she doesn't book, move on. Sorry to sound so harsh, but sometimes that's what we have to do! Good luck!