(This question only applies to Narrator 1; Narrator 2 uses different markers, and is much smarter about speaking.)

Narrator uses the color of the text to match text to actors. The pasted text must use different colors than in the actors drawer - even if it looks the same, it might be slightly different. To correct this, do the following:

1. Paste in your text;
2. Select all (Cmd-A);
3. Click on an actor in the drawer, e.g. Default;
4. Click Apply.