When used with a job where concurrent builds are enabled, the s3 plugin waits for the completion of previous builds before commencing on the current build.

This might be similar to JENKINS-9913.

The log output at the end of build #2373:

02:51:12.281 Publish artifacts to S3 Bucket is waiting for a checkpoint on Run crunchie job #2372
15:24:05.276 Publish artifacts to S3 Bucket Using S3 profile: hudson-key
15:24:05.442 Finished: SUCCESS

As can be seen, it's waiting 13 hours for the completion of #2372 (until I killed #2372, at which point the s3 plugin completed successfully).