I left my phone in an Uber, but the driver refuses to give it back. What do I do? Uber seems to ignore me.

Are you sure the Uber driver is receiving your messages?  Before you assume he is ignoring your messages, check with Uber.  I believe there is a set number of hours your messages and calls will go through to your past driver. 

Also remember all the messages and phone calls go through a switchboard.  All the phone numbers are changing - constantly.

This happened to me once.  One of my customers was apparently furious and made all these claims to Uber because I wasn't "answering my phone."  She thought she had my cell phone number and tried to harass me.  Go figure. (crazy person)

I don't know the exact contet.  I feel like you built up some bad rapport with the driver if he is refusing to give it back or to make an arrangement.  Did you do something to make him mad?

...or is he just saying he can't find it?

Either way, calmly message the driver you'd appreciate it if he can meet up with you for a predetermined tip amount.  Time and distance driven are money to them.  Make sure to offer an amount that compensates for his expenses.   If you go through Uber, they'll automatically charge your credit card and the fee will be passed on to the driver.  However, even that isn't enough sometimes.  It's your phone, it's worth hundreds of dollars, and after all it is your fault.  Offer a reward and pay his expenses.  

Also don't forget the opportunity cost.  He has to stop working to deliver your phone.

This is definitely a form of theft.   It isn't the driver's legal obligation to deliver the phone, but he has to make a reasonable attempt to get the item back to its rightful owner.  If he doesn't want to deliver the item, he should at least drop it off at a Uber office or the police. 

From your question, I can tell there have been some communications back-and-forth.  Just provide your demand, whether it's a financial reward or instructions to return it to the police.  Make sure to record everything, including all the messages and the driver info.

Sorry you are dealing with this.

Umm, this is theft and criminal.  Report it to Uber first, and if he still doesn't comply, report him to the police.

Did you try finding the phone's location on the GPS?  Show up at his house and demand it back.