A horizontal line can be a result of some dust or dirt collecting on the surface of the print head. This can be corrected by cleaning.

To clean the machine:
 

1. Turn off the P-touch.
2. Open the battery compartment cover on the back of the machine and remove the batteries, or disconnect the AC adapter.
3. Open the tape compartment cover and remove the tape cassette.
4. Gently wipe the print head up and down using a dry cotton swab.
   
5. Use the cotton swab to wipe up and down along each roller while rotating them with your finger.