First off, make sure you have the Bug issue type available in your Jira project, so Shake can create Bugs in your Jira project.
Required fields in Jira
Also, if you or your Jira administrator made some fields required to create a new issue (e.g. Components , Estimate , Epic , Platform and so on), Shake won’t be able to forward bugs to your Jira project because bugs arriving from Shake don't contain that data. You will receive this message when you try to forward Shake ticket to Jira manually:
Jira didn't accept this bug. We suggest you to try again a bit later.
To resolve this, change settings for creating an issue in your Jira project (or contact your Jira administrator to do it for you) so those fields are not required any more, but optional instead. Once you're done, go back to Shake and try integrating your app with Jira again.
This always helps. If your problem still persists though, click the bubble in the bottom right corner and let's chat. We'd like to help you!