I'm trying to send a session variable from one flow to another flow, through HTTP endpoints.
The session variable set in first flow is not available in second flow.
2 answers

Session variables can be easily propagated from one flow to another through the VM transport, or a flow reference, but not through the HTTP Connector.
Please refer the Mule documentation for Session Variable Transformer: https://docs.mulesoft.com/mule-user-guide/v/3.8/session-variable-transformer-reference