First of all, nice job! It reminds me of the only use I had for Ettercap when I was a little cheeky b..oy :) poison other people's IRC conversations. It was great fun!

> do you think it's because of the packets loss % or something else can cause that ?

Potentially, but I think a network capture on both victims would surely explain it. At some point you might be even getting some asymmetric traffic: packets from A to B go through you, but from B to A don't, until the ARP cache is poisoned again.

I'm not a coder (or a shitty one :)), so I can't infer from your code things like how often do you renew your poisoning.