Are you sure? The monopoly bonus shows up if you hover over the gold icon at the top. It's really not that difficult to achieve. In fact I would argue it's harder to somehow play hundreds of games without achieving one.
By default you only need 60% of a given resource to gain the monopoly bonus. You can change this value with Monopoly++ mod.
Do you not have other mods? Something could be incompatible.
You can ask in the steam page, but I didn't notice any issues playing with Monopoly++ after the patch.