Passing Transactions are missing for few Linux servers since June'17. Passing Transactions Tab are empty there is no data to investigate or check. Even Transaction Flow Tab also not helpful so please advise to resolve this issue on prior. Refer the attached screenshot.
First thing I would check is that your agents on those boxes are still instrumenting the right processes and that they are showing up as connected with no issues (green status icon). Apart from that there are a number of potential issues that would be a bit more difficult to diagnose. I'd check the agent, collector, and server logs to see if there are any potentially related errors and also try and determine if any changes (either to the application or possibly the network - firewalls modified or added or other networking gear that could interfere or modify the data being sent by the agent) may have occurred around the time you stopped seeing traffic. Unfortunately given that it is several months ago that may be more difficult.