If you look into your STP (Single Touch Payroll) mailbox and notice a message with a Failed status, it may seem concerning but it almost never is a problem. There are several message statuses that you may see in the STP mailbox:
Transmission Successful
Transmission Unsuccessful
Sent/Awaiting Response
Failed
The STP button colour indicates the status without even going into the mailbox. If a message is marked as Transmission Unsuccessful , this is a concern and Lightning Payroll will prompt you when you enter the Pays screen if the issue is still unresolved. A Failed message on the other hand, does not mean that the message failed to send to the ATO, it means that a response (I.e. successful or unsuccessful) was not collected from the ATO. Lightning Payroll automatically checks for responses upon shutdown for the desktop version, and overnight for the online/mobile versions. You can manually check the response yourself by going to Pays >> Single Touch >> Single Touch Mailbox a minute or so after sending. These responses are only able to be scooped up within a few days. Once that window is closed it cannot be retrieved. If the failed is followed by a Transmission Successful you can safely assume the Failed message too was successful.
For peace of mind you can also check your STP submission history by logging into the ATO's Online Services For Business and clicking on Employees >> STP Reporting.