Native vlan means that the trunk will carry that vlan WITHOUT a tag. I recommend that you try always to put a tag to every vlan or type of traffic. An important point is that if you use native vlan in that trunk, you also need that the router doesn't...

Can you verify if the HP L3 Switch, know about the network 10.1.204.0 , 10.5.204.0 & 10.6.204.0 ? I think that the problem is that the return traffic can't reach this networks because this HP L3 switch don't know about them.