Hi,
As far as I know there are two possible reasons:
1. The design contains a lot of pieces (over a hundred). The software will have difficulty finding the correct order simply because there are so many possible combinations to consider. In this case you may want to wait a bit longer (e.g. a couple of minutes) before giving up.
2. There is some sort of flaw in the design that the software stumbles on. In this case it will run around in circles and never finish its attempt to name the pieces.
If you can't find the cause and want me to look at this specific case, please sent me a private email with the design.
Arnout