Are the expressions attached to the staff they appear to be? I find hairpins in particular sometimes look as if they are attached to one staff when they are actually attached to the staff below.

… When I attempt to extract parts, the extracted part does not contain what is in the score, I created.  There are missing measures …

Take a good look at the linked part.

My first guess would be that the measures are also missing in the linked part.

The culprit is probably that multimeasure rests have not been broken, as new notes were added in empty measures.

Try this in the linked part:

Create multimeasure rests (again).

Then, extract the part (again)

By The Way:

Are you sure you need to extract parts?

You can not use the linked parts instead?