Is the CorrelationID passed correctly? Revisit the mapping for the Send and Receive worksteps; check the value in the CorrelationID dataslot after the Send workstep completed.
Did you define the message formats correctly? The format of the outgoing and incoming messages should match. Check the adapter configuration for both Send and Receive worksteps. If you find and correct any errors in the adapter configuration, make sure to run the map configuration again for each workstep, otherwise your fixes may not be active.
Confirm that the Wait for adapter to complete before processing checkbox setting is correct. The box must be checked for the Send workstep, and cleared for the Receive workstep.