Cannot copy all the files from one folder to another

Hello
I'm in the process of converting quite a few files from WMA to MP3 via MediaHuman Audio Converter which allows me to do it in bulk rather than one at a time. The MP3 files are created in a folder on my hard drive and I'm trying to move them to a Sansa music clip connected to a USB port. Only a few files are copied to the Sansa folder, for example 6 out of 13, and when I try to add others I get a sound but no error message. I tried deleting the receiving folder and creating it again but same result. I tried to empty the clipboard but same result. I tried it on different folders but same result. I delete the WMA files on the Sansa so there is plenty of space for the MP3 ones.
I'm using WIN10 and couldn't find any explanation for this type of situation on all the sites and forums I consulted so any help would be greatly appreciated.
Thanks,
Bobhrx

Bobhrx said:

Only a few files are copied to the Sansa folder, for example 6 out of 13

Are they always the same files? I've had problems with Sansa Clips because of the filenames - either too long and/or special characters.

Also this shows up repeatedly in google for Sansa and other music player devices with missing file problems:
Sandisk Support - MTP or MSC

Thanks for the quick answer.
It happens with all the files I try not just the same ones.
I tried shortening the file names and taking out special characters but that didn't solve the problem.
I'll keep "messing around" with it and see if I can figure it out.

It looks like the reason the files wouldn't transfer is that the Sansa was full but the storage numbers displayed were not correct leading me to believe that there was more free space. I manually calculated 1 file at a time to figure it out.
Thanks for the answers.