Hi Jennifer,
Since you have your most up-to-date information on your iPhone, let’s try resetting Codebook by removing your encrypted database on your Mac and restoring from Dropbox. Here are the instructions to reset Codebook on your Mac: Resetting Codebook follow the instructions under “For OS X”.
After which setup Codebook for Mac with the same master password as you were using before, then attempt to sync again.
Let me know if this allows you to sync regularly. Thanks!
Cheers,
Micah