If you are seeing various icons to the right of your prospects inside of your Sequence's Issues tab, as shown in the image below, hovering over the icon will give you insight into the issue for that particular prospect.
There are numerous issues that could arise with your prospects inside of a sequence, and the message will let you know the reason why the prospect has an issue.
Depending on the message, there may be different courses of action that are needed to resolve.
For a deeper dive into the Issues Tab, how to remedy, and what the icons means, please check this out.