If roaming mode is set to automatic it should automatically go into roaming mode whenever necessary. Verizon is ONE of Sprint's roaming partners and if a Verizon tower was available, it should have locked onto it.
How long have you had this phone? Are you sure that roaming is available on your plan (you can have a no roaming setting added to an account. Some people do it...for whatever reason!)?
You may need a PRL update, depending on the age of your phone and when you received the last update. You can call tech support and ask for one. They will send it to your phone and walk you through completing the process. It's not guaranteed to work but, in some cases, it can resolve the issue. You have to call to get it done though, you can't do it yourself on your phone.
If that doesn't work, tech support should be able to do some basic troubleshooting with you to see if they can get the issue resolved. If not, the device might be faulty and need to be replaced.
Hope this helps!
