backup performance

[[EMAIL PROTECTED]]

Hi,
we  use oracle TDP with LTO 2 dirvers for Rman database backup.
Normally, the backup for database (450G) take 2h with 2 channels. but some time 
(intermittent) the backup take more than 2h30.
the problem is that in this case only on channel work fine but the other is 
very slow, so when we do recovery test , the multiplexing not work.
any idea please ?
 Thanks