What happens when restarting Store and Forward

Here's our situation

A: Data Collector Computer(CSV collector) -> Store and Forward

A sends data to computer B

B: Historian Computer receives data through Store and Forward

Between A and B, TCP port communication, everything is ok. 

But sometimes, I can see CSV files are going to Store and Forward buffer in computer A, although there's no port connection issue.

So I go to Services tile of Computer A and, click 'stop' of Store and Forward, and Start again.

Then buffer count starts to decrease, sending the data again.

I'm curious about what happens after I restart Store and Forward services of the collector machine, as it does resolve the problem.

This kind of issue had never happened until we started installing tunneler and, some network related programs. I believe it's affecting the Store and Forward but just don't know how, and how it is resolved after restarting the store and forward service.

Could you share any insights into this?