I've had an epidural as post operative pain management and it hasn't worked. is that normal? can it be related to my thoracic syrinx?

Possibly: Epidurals are not 100% successful. The syrinx could be related, but more than likely it is not the reason.

Can happen: If the epidural catheter was not placed correctly or if it migrates out of proper position it will not work correctly. I doubt any correlation to your syrinx.

Unfortunately: There are many reasons, technical, anatomical, and pharmacological why an epidural doesn't work. The syrinx could definitely play a role in the poor performance.