They both work by the same principle. The idea is that the convalescent serum provides virus-neutralizing antibodies, and the monoclonal antibody works that same way. The advantage of the monoclonal antibody, of course, is that there's better quality control, so you know exactly how much antibody you're providing. With the convalescent plasma, there could be enormous variability. That's why you're getting very inconsistent results, as well.
If you have very high titre levels of convalescent antibody, it could work quite well, but a lot of places don't adequately measure it, so there is all that variability. Of course, the problem with both of those products is that you have to give them very early on in the course of the illness, when you're still interfering with virus replication. Remember, there are two components to COVID-19. There's the virus replicating phase, and then there's the host inflammatory response. Once you delay and allow that host inflammatory response to continue, it's clear that the monoclonal antibodies and the convalescent serum are not working very well, so you have to give it early on in the course. It's certainly no substitute for vaccination.
I don't quite understand why monoclonal antibodies are not more widely available. In the U.S., too, there's been a problem. For instance, when my daughter-in-law got COVID-19, she was living in Arizona and wanted to get her monoclonal antibody, and the infectious disease attending at the medical centre there gave me a list of about a hundred criteria why she couldn't get it. They've made it so fussy and complicated and have limited the criteria so that, at least for the last few months—maybe it's gotten better now—it was almost impossible to actually get it used for anybody.