No heartbeat found problem with HKPILOT32

I have a pixhawk from HobbyKing (HKPILOT32) which works well except …
I can not update the firmware using MissionPlanner. The board is detected as pixhawk, firmware is downloaded from internet and then the board does not respond and I get “No heartbeat found” message. I tried several “tricks”: disconnecting GPS and telemetry radio, booting with safety switch pressed, power with external module … but it didn’t help. Apart from this, the board seems to be OK (copter flies well, MissionPlanner works flawlessly for telemetry etc).
Does anybody know the solution for this particular problem?