Trunk doesn´t work

[MDADV Technik]

We had changed our backbone Switches to Brocade 4900 with FOS 5.2.0a.
HP recommends to set the parameter in our fabric dls not set (dlsreset) an the fabrid.ididmode to 1.
We had 4 Trunkmembers and all the licenses enabled.
In the output of switchshow all trunks look good, but during nightly backup one trunk member is at 170 MB/s the other nearly at 0.
Why aren´t they nearly balanced?????????

Are there any ideas?

Regards peter