Singer 338- raising presser foot does not release top thread tension

I have just acquired a beautiful singer 338. I have cleaned it up and oiled it and it's working just great. The only problem I seem to find with it is that the thread tension is not released by raising the presser foot. I kno w the mechanism works by pressing against the tension assembly where the li ttle peg enters the hole into the case. But I'm unable to see any parts mov ing in that vicinity when I lift the presser foot nothing is engaging with the tension assembly to release the top tension. Does anyone have any exper ience with this? How would I go about fixing this? Thank you in advance for your help.