Now that I think of it most of the times mandates do not get completed when the requirements are met. One term a mandate gets completed the next term the same mandate does not get completed even when it says "2/2 mining districts built". (also it would be nice if the reward was 1 years' of unity and uncapped so that it would be equally powerful to agendas)
I've noticed the "increase surplus to X" mandates specifically are not firing. Some of the others are for me (e.g. build districts, build stations)