When trying to enter a third waypoint into FMS for the 737 I get nothing but "Invalid Entry." I'm simply trying to put the third waypoint in on a short route from DFW to Austin. The flight plan is TTT to WINDU to CWK to BLEWE. However, on the FPLN page 2, I cannot enter past the second waypoint, WINDU. When I attempt to enter CWK and then hit 'To' I just get the 'Invalid Entry' response. Why is that?