There are a few additional settings which enable some addition options when signing off. Under Jira administration click Issues and then click Settings under the Sign Off heading.
If you would like to be able to mark a sign off step as Failed you should enable the option for Failed Status.
In Progress Status
If you would like to be able to mark a sign off step as In Progress you should enable the option for In Progress Status.
Un Sign Off
Once a step has been signed off, it cannot be changed. If you would like to allow a step which has been signed off to Pending, Failed or In Progress, enable the option for un sign off.