This happens if you directly edit the existing text object at the top left-hand corner of the first page of the full score or any of the parts, which you should never do.

This text object, which says Full score by default in the score, and the name of the instrument(s) included in the part in the parts, uses a special wildcard to achieve this. Wildcards are special pieces of text that can be automatically substituted by Sibelius with data taken from elsewhere -- they are discussed in detail under the heading Wildcards at the end of 3.1 Working with text in Help > Sibelius Reference.

The wildcard used for the text at the top of the first page is \$PARTNAME\, which takes its value from the Part Name field in the File > Score Info dialog.

So to fix this, you simply need to repair the text object:

• Switch to the full score if necessary by hitting W
• Double-click the text object at the top left-hand corner of the first page to edit it
• Delete the existing text and replace it with the string \$PARTNAME\, then hit Esc to stop editing
• Notice that the text now says Full Score
• If the text is shown in the score but you wish it to be hidden, choose Edit > Hide or Show > Show in Parts

If you want to change this text in any of the parts, the secret is to edit the Part Name value in Score Info. There are two ways of doing this:

• Either single-click the name of the part in the Parts window and type the new name, hitting Enter when you're finished; or
• View the part in question, then choose File > Score Info and edit the name there.

By default, Sibelius shows instrument names at the top left-hand corner of the first page of the parts and in the center of the top of subsequent pages. If you can't see them, try these steps:

• First, make sure View > Hidden Objects is switched on; then look at the start of the score. The instrument name should be visible, either in gray (meaning that it's hidden) or black (meaning that it's shown)
• If the instrument name is there but grey, select it, then choose Edit > Hide or Show > Show in Parts, which will hide it in the score but show it in all the parts.
• If no instrument name is there, then you've somehow deleted the required text object. To put it back:
  
  
  • Open Window > Parts and click on the New Part button (it looks like a blank piece of paper).
  • Select all the instruments listed under Staves available, then click Add to Part.
  • Verify that all the instruments have moved from the Staves available to the Staves in part column, then click OK.
    Creating this new part reinstates the missing instrument names.
  • Select the newly created part (containing all instruments), then click the Delete button (it looks a rubbish bin).
    This simply removes the superfluous part once the instrument names have been reinstated.

By default, Sibelius doesn't show instrument names to the left of each staff in dynamic parts, because parts normally only contain a single staff. However, in rare cases you may need to have multiple staves in the same part, and you may then wish to show instrument names at the start of the first (and perhaps subsequent systems). To achieve this:

• View the part in which you want instrument names to appear
• Choose House Style > Engraving Rules and go to the Instruments page
• Set instrument names to appear in Full at Start of score (and Short subsequently, if you wish), and click OK

If you have many parts in which you need instrument names to appear, rather than repeating this process in each part individually it may be quicker to export a house style from this part and import it into the others:

• While viewing the part with the desired instrument name format, choose House Style > Export House Style
• Type a suitable name for your house style and click OK
• Open Window > Parts, select the parts in which you want these instrument names to appear, then click Multiple Part Appearance; click No when asked if you want the changes to apply to all parts rather than just the selected ones
• On the House Style tab of the dialog, click Import House Style and choose the house style you just exported, then click OK