There was a period of a few years when SOCOM was diligently tracking and interdicting shipments going from Iran to Yemen, but I believe most of that work has been passed off to other countries. Here’s a presentation by CENTCOM on the issue: U.S. Dhow InterdictionsI have to give them credit: they've tied down an entire carrier strike group using cheap drones and the odd missile here and there. I'm curious how Iran is moving all of this material into the country year after year.
It’s likely that the Houthis, recognizing that the flow of advanced weapons from Iran wasn’t going to meet their needs, began internally manufacturing some of the systems we're seeing used today, with technical support from the IRGC. I’m not sure if these current systems being used are Iranian imports or domestic manufactured versions.
This is a pretty good read if you want to learn more: Houthi anti-ship missile systems: getting better all the time