Not sure if it is bugged.
Parks, Education, and Transportation are capped at a 100% boost each. Entertainment, Casinos, and Landmarks combine together for a maximum of 100% boost, meaning the max pouplation a building can get is 400% boost.
A ferris wheel already adds 60%. If you only had ferris wheels and added a second one, your population where it overlaps will only go up by another 40%.
See this thread for the info: http://answers.ea.com/t5/SimCity-BuildIt-General/Wealth-Forecast-Luxurious/m-p/4228654#M709
