Hi and welcome to the Chiari forum.
As KerriC 1 mentioned it does indeed sound as though you have yet another CSF obstruction and could be scar tissue.
May I ask, were ALL related conditions ruled out prior to surgery? And what type of dura patch was used?
Next, as KerriC1 asked, do you have a Chiari specialist?
It's possible you have something other then just scar tissue causing the obstruction and only a Chiari specialist will know to look for them.
Yes it sounds to me as though your flow is blocked again. Posteriorly is exactly where chiarians have that problem. I’m no dr of course, just went through 2 surgeries myself.
My second was due to significant adhesions and scar tissue completely blocking the flow. And I wonder if that could be an issue for you also
Do you have a chiari specialist?