Show PDF image in Planner task not always working

Can anyone answer why sometimes I can see a PDF in the Planner Task when I 'select to show card' and others I just get the PDF logo.    I have uploaded the files into the Teams folder structure and then selected to attach the PDF to the card via 'SharePoint' (so that I can control where the file is located).      I'm using Teams on my laptop and doing this with all my PDFs, some show on the card and some don't.   If I open in SharePoint I see exactly the same thing so viewing it there is not a fix.   If I chose to attach a file from from an upload it dumps the file in the main documents folder which is messy, so that is why I want to attach it via SharePoint.   The only thing I can think that is different is that the PDF files that show an image are within 1 folder level down under 'documents', and those that do not show are within 2 or 3 folder levels down in 'documents'.    Any thoughts?