For ps2 to usb there are 2 types of adapters, some are passive and directly feed the ps2 clock and data lines on the usb data- and data+ lines and let the keyboard controller deal with it, those can be used with some keyboards that are meant to be used that way mainly in that transitional era from PS2 to usb.
The 2nd option are active adapters which take the inputs from the ps2 keyboards and actively translate them into usb, this kind should be compatible with pretty much all ps2 keyboards (model Ms and Fs can sometime crash the weaker ones).
So if you are using a passive one, your problem is either you are using a green one (meant for mice, different PS2 pin-out) or your keyboard is not compatible, if you are using an active adapter for me it would be that the adapter is either dead or very special and requiring a special driver (usually those things uses USB-HID, and the drivers are included in all modern OSes)
Hope this helps