Fatal Error

hi there,
i'm using snapshot plugin which worked fine for a while but now it doesn't save archives properly.
however, it saves files when i've checked through the ftp.

what's happening is i have 3 snapshots one of which works properly, the other one has only two backups from the last month (it's set to 'once daily') and another displays this message instead of archieves:
Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 524288 bytes) in /var/sites/b/bandcloud.co.uk/public_html/wp-content/plugins/snapshot/lib/snapshot_utilities.php on line 781

these are the other info you might want to look at:

Bandcloud Version 3.4.1
PHP Version 5.3.10
MySQL Version 5.0.85
Is Multisite Yes, Number of Sites
Folder Permissions Writable (0775) – /wp-content/uploads/snapshots
Writable (0775) – /wp-content/uploads/snapshots/_backup
Writable (0775) – /wp-content/uploads/snapshots/_locks
Writable (0775) – /wp-content/uploads/snapshots/_logs
Writable (0775) – /wp-content/uploads/snapshots/_restore
PHP runtime information
Display Errors 1
Error Reporting 4983
Magic Quotes 1
Max Execution Time (seconds) 60
Memory Limit 256M
ZLib Compression Off

thanks,
Adam