Did you have an MRI of your full spine before surgey? Unfortunately herniation can recur, sometimes for no apparent reason. My first decompression was a little complicated, but successful for about a year ... until all of the sudden my symptoms were back, and I was really hoarse and having way worse swallowing issues than I ever experienced. An MRI showed my herniation was worse than before my first surgery, and had caused a cervical syrinx that was the width of my spinal cord. I had surgery the next day.
So yes, a second decompression is possible. I would call your neurosurgeon and schedule a follow up MRI if you haven't had one recently.
In addition to what was mentioned above...scar tissue can also develop post op and create the same CSF restrictions as b4 surgery.
Other possibilities are a slow CSF leak......
Or it could be a related condition.....so do see a true Chiari specialist for a MRI and review to make sure you do not have a related condition and if you may be a surgical candidate again.
They can absolutely do the surgery again. The surgery you have for chiari is only a treatment, not a cure because it usually comes back after 15+ years. If your cerebellar tonsils have fallen down again, they can keep doing the surgery, but I wouldn't jump to conclusions just yet, you should schedule an appointment with your surgeon, he'll/She'll probably order another MRI to be safe. There could be tons of reasons that your symptoms are coming back and I wouldn't worry too much until you have more information.
I know it's hard. My surgeon told me I had "Vanilla" Chiari and I'll only need 1 surgery and i'll be fine for at least 10 or 15 years and I wish that is what had happened, but it's not. You can never be too careful so you should have it looked at before it becomes unbearable again, if it will be.