We seem to get this question enough now that a good and proper answer should be given specifically to this very simple issue.
When Google has a blocked resource it is generally a 3rd party script/resource or a local script/resource that accesses 3rd party components. You will see there are two images and the image on the right will generally have what you expect. You will want to check this.
This is not to be compared to how Google will render your site when fetching your page normally. It is a bit concerning I agree. It may be that these 3rd party elements are excluded to make the render option faster. Not sure. It would be best if we knew the case absolutely. Remember that the Render portion of Fetch and Render is still relatively new and it was only a few months ago that rendering was a bit buggy even for simpler sites. I rather suspect that while there is code overlap, the two renders are developed separately. The reason is that the Render portion of the Fetch and Render is likely smaller since it is not tied into the index.
You will want to make sure that the resource is consistently available.
If resources are blocked, Google gives you a notice. It is a notice just for that Google web page and not an error on your site. You generally have nothing to worry about or fix. It is just a notice to explain the results of the Google page. As long as your site works for users, you have nothing to worry about.
Admittedly, this should be a lot clearer.
