The first thing to check is the dimensions in pixels of your image. If you’re seeing the image file marked as 300dpi, it’s important to know that that is just a piece of metadata, and doesn’t actually impact how the image appears on the page -- DPI can be very misleading! If you have an image created at a resolution of 300dpi, it is only 300dpi at one specific size.
If you’ve checked the dimensions of your image (you can do this by going to your media library and clicking on the relevant image to see the details) and they are in fact large enough, the next thing to look at is the size at which your image was inserted. You can do this by clicking on the image, then on the grey pencil icon that appears to open the image editing screen. Here, you’ll see an option to change the size, which should be full size.
If your images are still being flagged by your distributor as under 300dpi, you may need to insert a custom class for your image size. Follow the instructions available here: https://pressbooks.com/blog/preparing-and-uploading-your-images-for-pressbooks-2/
If both of these things are in order but your image is still being flagged as less than 300 dpi, please contact us.