Do you have a link to the page in question so we can properly see what you are doing?
You do realise that background-attachment fixed places the image in respect of the viewport and not the element you applied it to? You will only see the background in the element when your element passes over the place where the background is placed. If your element is only content height then the image is only seen in the content part of your element and while that element is over the position the fixed attachment has been placed.
As Ray suggests it sounds as though your element has only content height which means you will only see part of the image assuming the element is over the position that the image has been placed.
If you place the fixed attachment on the body element it will cover the whole viewport and be available at all time if that’s what you wanted.
However without full html and css that’s just a guess