I'm a new KDE and Amarok user, and I know it's annoying to start at the forum with a question, but here it goes:

I've imported my music to Amarok, and it worked fine. But yesterday I put 4 more folders into it and they don't show amog the others. At first they did, and then after I edited the files information (artist name, album name, etc.) they disappeared from the collection. I made Amarok re scan my music folder, but they still don't show.

If anyone can tell me what I'm doing wrong, I appreciate it.

Thanks.

Could you please state which Amarok version you are using? Current Kubuntu 10.04 comes with Amarok 2.3.0, what you describe sounds like an old bug in a 2.2.0 version that is solved since quite some time.
Also make sure you have the option "Watch folders for change" activated, and restart Amarok after a rescan to make sure the Collection browser is properly updated.