I suspect the problem lies with the formatting in the master file, not the formatting in the chapter. If the formatting in the master file defaults to single space, then that's going to affect everything else.

Try opening the master file, setting its default spacing to double space, save, and close.

Then try re-opening the chapter you were working on.

If that doesn't work, I'd be happy to take a look at the files to see if I can figure out what's going on.