There are some steps required when you add an extension for recording, please see forum post:
Add extension: Trunk side recording - Avaya PABX and CSTABasic summary of steps: To play the recording back, you need to add the extension as a recorded devices(else it will be rejected), the extension must get call details(avaya csta wil provide it, if enough licenses), a user name must be setup for the extension(in AMETHYST webadmin under 'Configure Virtual Extension) and your supervisor login must have access to that user name(in AMETHYST webadmin under 'Configure Supervisor'.