Flexsim seems to have difficulty exporting some flowitems to USD. I think its all the flow items that has another shape frame selected than the base frame. Its kinda a shame, cause it othervise looks reeeeeally nice in omniverse. In the example below it looks like that if the flow item is on another flowitem (subnode) it just dont show up.
If its on a transporter, like the agv in the right side of the image, it just shows up as a white cube, or at least sometimes, I cant seem to figure out the logic.:)