Connection failure at a particular route

Please help me. I have a problem.

I replaced the FW from RT.

The FW is FG-300D-BDL-US, and FW is HA cluster.

SV3 received files from SV2 by FTP.

First, No problem. But, after 4-5 hours, SV2 cannot send files to SV3.

SV3 received date from SV1, it is no problem after 4-5 hours.

I have no idea why SV2 cannot send files to SV3 after 4-5 hours.

What would be the cause?

(OLD)

SV1   | ----------------------- NW1                        |    SV2               RT     |                  | ----------------------- NW2            |           RT    <- Replaced            | ----------------------- NW3     |    SV3

(NEW)

SV1   | ----------------------- NW1                        |    SV2               RT     |                  | ----------------------- NW2            |           FW x2    <- New!            |  ----------------------- NW3     |    SV3