I’m only just starting to get into gardening and we have a lot of garden waste to try to deal with (including a lot left by the previous owners). In the last few places I’ve lived, the previous occupants have always left a big pile of garden waste and it’s just sat there. Turns out you actually need to put in some effort to make compost - it’s really a big organic system that needs nurturing. Also, it does take a long time; months to years, apparently - but I’m not quite there yet so I’m no expert.