This can happen if you have been using CC since before version 3. You need to tell CC to ask calibre for more detailed covers. Do this requires 2 steps:
- Go to Settings (tap the 3 dots on the upper right of the screen then choose Settings), scroll down to Covers and Thumbnails, then change "Cover size in book details" to Medium or Large. Changing any other of the cover size options will work as well.
- Connect to calibre as a wireless device. Calibre will then send the more detailed covers to CC. The covers will appear fuzzy until this connection is completed.
Please note that sending the new covers can take some time. The first connection after changing the "Cover size in book details" setting will be slower than normal.