Howdy Ian,
I just tried the following steps to successfully create a 40 measure piano score and then add another 16 measures to it:
1. Open Composer
2. Click "Create"
3. Step 1 - select "Piano" and click "Next"
3. Steps 2 and 3 - click "Next" (default of 4/4 and key of C)
4. Step 4 - enter "40" for number of measures and click "Next"
5. Step 5 - click "Finish" (no song title, Window view, and box at bottom is unchecked).
6. (now have a 40 measure two-stave piano score) Click "Measure" in the menu bar.
7. Select "Insert (add) measures"
8. In the dialog, I entered 16 in the "Number of measures to add or insert" box, and clicked "Ok."
Composer properly added 16 measures to the score without adding any extra staves. The steps above are minimal - I did no editing of the score at all. Was there any editing that you had done prior to adding the measures, or did you perhaps earlier add new staves, but then decide you didn't need/want them, or....?
Any additional information (or a copy of the "before" file) should help us figure this out
ttfn,
Sherry