Strange behavior port monitoring 6850/9800

[Vektor]

Hi, I'm trying to configure a port monitor session on a remote 6850 (AOS 6.4.3.640.R01). Putting the same configuration on several switches (but with the same AOS) sometimes I have weird issues.
With command line: port monitoring 1 source 1/6 enable, I should sniffer all the traffic saved with a file .enc on the flash but;
On a 9800 I can't use the bidirectional feature (the default one) as I can't cath traffic. If I enable the inport option, I have the .enc file
Trying to put the same line on several 6850, I have nothing as output, no file in flash and no error from "show port monitor status"
On another one (6850) with the basic command line port monitoring 1 source 1/6 enable , no problem.
Any idea?
Thanks

/Stefano