Please attach the SVG if you would like specific feedback.
Sounds like you are using the right tool for the new line and the wrong tool was used for all of the original lines.
This link kind of explains it:
If that is unclear, you can make a small stroked path with a big stroke width, and select it, and zoom in close and use the expand command to see up close what happens.
Thanks for sharing your results
-Mike (videoscribe user)