hi i am new to vapi ai and recentl starting working on a custom llm with vapi. i have exposed my server to vapi using ngrok and i recieve the first message from vapi ai and after that the agent goes completely silent even though i send the response back i am using llama 3 8B model which is hosted on openrouter and made sure that the response structure format is the way it should be but still vapi ai stays completely silent after the initial greeting does anyone know the solution to this?