Ahh yes...I have had problems with PS/2 to USB and back in the day serial mouse to PS/2 and a third type of unknown old keyboard connector to PS/2 or USB. I finally found out that the only adapters I could count on were the ones that came with the keyboard. You could continue trying adapters (I have a jarful LOL), or see if you can find one on the keyboard manufacturer's site or email their CS staff to ask what brand of adapter is approved to work with their keyboard.
Tony
"Buy what you like, or you'll be forced to like what you buy"...me