Pry off the key and spray some electronics Contact Cleaner down the hole of the Alps switch. The general sort of thing you will find can be seen here. Contact Cleaner should be your first port of call in my view; it has worked for me, on Alps switches that worked intermittently or only with force, or indeed not at all.

Failing that, Google and ye shall find somewhere or other how to fix one of the switches by more drastic measures.

I have collected several of these recently and they are really excellent keyboards that are well worth fixing. The switches do, however, have a tendency to need a little TLC after all these years.