Setting joint velocity through plugin

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I'm writing a model plugin in C++ to use with ROS+Gazebo via python (similar to AtlasPlugin) I wrote a reset function to allow me to set initial conditions for a model (world position, joints positions and velocities). Everything is working right except for the joint velocities. I'm using Joint::SetVelocity() but nothing happens. I also tried using Joint::Update() after and still nothing... Here's a small snippet of my code:

this->joints[i]->SetVelocity(0,_req.joint_vel[i]);
this->joints[i]->Update();

I'll appreciate any help :)