I can see that your product #0003 may have trouble stamping, as the file for that product is over 35 MB in size, whereas our PDF Stamper has trouble stamping files larger than 20 MB or so. That file appears to have a cover page with a high-resolution image that has not been resized to fit the much smaller page where it's displayed, so that is inflating the size of your file unnecessarily, not to mention making the file hard to open (it takes my computer over a minute just to display that first page).
Generally when saving a PDF to be distributed online, it's a good idea to "optimize" the PDF to make sure the final file isn't any larger than it needs to be. Adobe software usually offers PDF presets that say something like Optimize for the Web, Optimize for Smallest File, or Optimize for Mobile, so look for any of those when saving or exporting your PDF from the program where you created it. If you can optimize the file to be under 20 MB (and the smaller the better), you can edit the affected product in our Seller Admin and use the Replace Product File button there to upload the optimized file in place of the original. It looks like your file was created with Adobe InDesign, so I found this page you may find useful:
Which other product had the trouble with being stamped sometimes but not others? Bear in mind that after you upload any new file, it may take up to 15 minutes before that file can be stamped, so please wait at least that long before issuing a test link. This and other tips and details are covered on our main PDF Stamping help page here: