Perhaps you're both sitting in your car right now, comparing what's being shown on your phones.
Your times match. Your destinations are the same. You're sitting next to each other, so location shouldn't be a problem.
What's the deal?
Give it one more try, but before you do, remember one thing:
The app will only ever match your ride with other rides that haven't already departed.
Makes sense, since it's hard to jump aboard once the boat has sailed. But it's also an easy thing to forget when you're booking that last-minute ride with your carpooling pal, trying to arrive as soon as possible.
You might be caught up in a vicious cycle right now: you've entered your arrival time, but the length of your journey pushes your departure dangerously close to "right now". By the time your companion tries to match with you, your ride has slipped into the past, and so it goes on ...
So try once more, but this time push your arrival back a little further - even 5 minutes (it usually won't matter if you actually arrive a bit early). Just enough to end up with a departure time far enough in the future for your carpooling buddy to match with you.
Of course, if you''re still having problems we can always help you at firstname.lastname@example.org