Take this example: Your contact is "John and Suzy Smith". The label for the invitation to the party includes only John, but you can see Suzy in the contact record: She is there.
Or in this example, Dr. John Smith, MD will show Dr. as his first name, or MD as his last. This can be very, very frustrating when you look up last name Smith and he is not there!
Possibly ACT! has recognized Suzy as John's middle name, and the labels are not set to print the middle name.
Here's what you do: Go to that ACT! Contact Record, and see the top left of the record, the box labeled "Contact". On the right side of the box, there is a smaller box. Click it. This shows the "Name Details" for the record. Select the proper portion of the name from the drop down list for first name, middle name and last name. For John and Suzy, select "John and Suzy" for the first name, and nothing for the middle name.
And "Presto-Chango", your invitation label will be correct! Now you can move on and find Dr. Smith!