Thank you for your comment, phred!
Here're the answers to your questions;
1. I'm using the version 3.4.8033.
2. MB was installed via installer (
https://www.getmusicbee.com/).
3. It's always the same songs that don't show up on my Walkman playlist. They're some of the newest additions to my library.
4. Here's how I send files from MB to my Walkman: right-click on the file in MB > Send to > Device WALKMAN (E:).
Finally, I'm sorry I couldn't understand exactly what you wanted in the last request.
Would you clarify what you meant by "lines," especially?
Again, thank you for your help!