[Elecraft] puzzling panadapter behavior

[jsdroyster at nc.rr.com]

Recently when I switch on the power to our Kline (K3 P3 KAT500 KPA500) the panadapter screen shows a blank black waterfall.  After making sure that an antenna is connected I changed bands and modes and behold, signals appeared and were also present when I switched back to the original band/mode where it had been blank.  This is a new development, and I am not understanding what is going on.  Yesterday one change produced a waterfall but today it took several changes to get it going.  Does this make sense to anybody?  THANKS!
Julie KT4JR