Seeking help with my Felt IA (2020 disc model) seat post saddle clamp. I have what Felt calls their VariMount clamp and is supposed to work with all saddle rails.

I've been using a Specialized Sitero Pro on it (carbon square rails), and am now trying to install a custom saddle that has round carbon rails. What I am finding is that the saddle fits fine in the clamp, but the clamp won't tighten down on it securely. I can easily move the saddle around. Not sure if the carbon rails are just slightly too narrow..

Anyone else run into this type of issue? Pics attached, thx!