[Elecraft] What to check when FT8 won't transmit (K3, Signalink)

[Stephen Dubin]

Just to see whether I like it (I didn't), I set up for FT8 with K3, Signalink and WSJT on Windows 10. I installed Meinberg when I had trouble receiving. I made a shipload of contacts and decided to go back to CW. Recently my daughter (KM4UDF) who is recently licensed said, "Aw C'mon Daddy, show me FT8."  I used same settings as before -- as well as I can remember. I double checked using the "Wilhelm Doctrine."   "Time,is"  says my system time is good. Software has VOX on and K3 has VOX off. Receiving (on 40 meters) went well with many CQ's and QSO's reported on the program. When I hit the "TUNE" tab on WSJT, the red "TX" light will light on the Signalink box, but K3 will not transmit. Changing position of "POWER" slider on software doesn't help. Changing MIC on K3 made no difference.Antenna SWR reads 1.1:1, Signalink has not been unplugged from K3 since time everything worked. Other modes (CW, Sideband) work fine.
My main question is, "If receiving works OK and Signalink "fires" when TUNE button on software is hit, what (hopefully systematic) steps should I take to see why K3 does not transmit now; although it did before. Is this a special curse on folks who swear off FT8 and then try to return?
TNX for guidance to the perplexed. 73 de W3UEC (Steve)