Why do my wonders keep failing?

I really have no idea what is happening but only 30% of my wonders actually get completed.

I always try to pick the ones with production costs I can manage in a reasonable amount of turns (I play marathon, so lets say, 2 X a 'campus' at most.)

And then I might clear some land to boost production, and convert the land to mines.

and with that, it costs about the same as a campus in terms of 'turns....'

but it still fails.