Album - Unable to create a thumbnail for this item

Does anyone know why I can create a thumbnail size for albums and then not others? I get the error 'Unable to create a thumbnail for this item' even though the movies within the album have already got thumbnails. The permissions on all folders/files are the same.

This is exactly what happened. I have a folder with two movies, both of which have thumbnails. The album also used one of the thumbnails for display, so everything was working as it should. Next I imported three more movies locally, the thumbs all generated fine for each of them. So now the folder has 5 movies, all with thumbs. I deleted one of the movies, the first one, and now the album won't use any of the four remaining thumbs for album display. I try to edit the album and where it says "Thumbnail" it is blank. I try to put in the default size and it says Unable to create a thumbnail for this item.

If I go and add a new album to the gallery and import some files it works as it should and the album uses one of the thumbs for display.

So whats up with that, anyone?

Gallery URL = http://www.mysite.com/archives/gallery/main.php
Gallery version = 2.3.1 core 1.3.0.1
API = Core 7.54, Module 3.9, Theme 2.6, Embed 1.5
PHP version = 5.2.4-2ubuntu5.6 apache2handler
Webserver = Apache/2.2.8 (Ubuntu) mod_jk/1.2.25 mod_python/3.3.1 Python/2.5.2 PHP/5.2.4-2ubuntu5.6 with Suhosin-Patch mod_ssl/2.2.8 OpenSSL/0.9.8g mod_perl/2.0.3 Perl/v5.8.8
Database = mysqli 5.0.51a-3ubuntu5.4, lock.system=database
Toolkits = Ffmpeg, ImageMagick
Acceleration = full/3600, full/3600
Operating system = Linux 64-155-128-157.dedicated.server1.net 2.6.24-23-server #1 SMP Wed Apr 1 22:22:14 UTC 2009 i686
Default theme = matrix
gettext = enabled
Locale = en_GB
Browser = Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7
Rows in GalleryAccessMap table = 3
Rows in GalleryAccessSubscriberMap table = 91
Rows in GalleryUser table = 2
Rows in GalleryItem table = 91
Rows in GalleryAlbumItem table = 23
Rows in GalleryCacheMap table = 22