What happens when a key feature in a new software solution doesn't work as expected?

Have you ever encountered these situations:

• a key feature in a newly implemented software doesn't work as your team expects
• a key feature in a newly implemented software platform does not work as demonstrated by the Vendor
• a feature is much more limited than what you were sold

How have you worked around this?

How have you worked with the software vendor to improve important features that are not up to your expectations?

How did things turn out?