backup exec drive pool

[lotfi123]

hi gents

i have a problem with my drive pool in back exec 8.6

when the backup start it write in the first type of the pool (constitued of 3) and then it goes to the second but instead to going in the 3rd one it back to the first and it faild

if anybody know how to configure a drive pool corectly or have any good procedure for that so the backup can start with the first tape and finish with the last it will be great if no body no i'm fired

thanks for your help

bye

lotfi