NAT with CONNMARK

I'm using CONNMARK with layer7 for doing traffic shaping, and it works great.
My problem is that i want to SNAT packets marked with CONNMARK (actually i'm doing "-m mark --mark X -j CONNMARK --save-mark"), and no packets go through the rule "iptables -t nat -A POSTROUTING -m connmark --mark X -j SNAT --to x.x.x.x". I also tried using "-m mark --mark X -j SNAT --to x.x.x.x".
All the marking is done on mangle table PREROUTING chain.

What's the difference between MARK and CONNMARK?

I've also got another scenario with -j MARK on mangle table POSTROUTING chain, and SNAT and it works fine.

I've searching in the web, and it seems to be all fine, but it doesn't work.

Thanx a lot.