Converting from .usr to .gpx

I used GPS Babel to convert my lowrance USR files to GPX but when I import them only some of the waypoints transfer and I get a import failed messaged.

- Is there a way to edit the file to make sure it converts properly?
- Is there a way to break up the GPX file by lake or year or something so I could import one batch at a time to isolate where the problem may lie?
- I don't get any kind of specific failure report that helps me to isolate what is not importing correctly.

Thanks in advance for the response and help!

- Greg