Why am I getting low resolution warning?
PDF to Book
I ran into this problem recently while using InDesign for a Blurb book. I got a low res warning on many images even though when I checked them inside InDesign they were all 300 dpi. Turns out there is a bug in the Blurb preflight check. Here is the email from Blurb describing the issue. Based in their response I went ahead and orderd the book and the images were fine.
Response Via Email(David) – 12/11/2011 08:58 PM
If you are receiving a low-resolution warning on your PDF to Book preflight page and you have confirmed your images are at least 150ppi, you may have encountered a small bug in our preflight software.
You can verify this by seeing if any images on the affected page(s) have a rotation applied to them. If they do, you may get this warning. Not to worry, as we do not modify your content and your images should print at the resolution you originally sent us. So go ahead and ignore that warning and order away!