KVM error, blank screen, not send Keys

From the first moment we have the server, the KVM fails us, we get a black screen sometimes and sometimes you see the Windows home screen but it does NOT let you send Ctrl + Alt + Del to be able to log in , I restarted the IPMI several times, I contacted the technicians, I explained to them and reasoned that it has NOTHING to do with the status of Windows to be able to send the desired keys, which if it is in sleep mode should not affect since the KVM connects at the BIOS level and therefore the only server status to consider would be if it had no power supply. For RDP, the server responds perfectly to the login keys.
We have open the case CS3424166, there is no way for Spanish technicians to do their job because they do not know, I have already sent them 20 emails explaining the problem and always answer evasively, backed by links that I have already reviewed a million times and to no avail, can anyone who YES knows can help us? Thanks.