Can't get happiness higher than 99%.

My population stays around 98% to 99% as well, I think it might have to do with some buildings not being close enough to parks. I can't say for sure though.

Are any of them near pollution zones?

Well there's definitely an outlier somewhere, see if you can find one building that's somehow different than the others. If all of your buildings have access to parks, check to see if there are any roads that are yellow or red, that might have an effect (red definitely does, I'm not sure about yellow).

I can't see your city, so unfortunately that's all I can suggest.

I've had my city at 100%, but since the update it's been sitting at 99%. I don't really know why I dropped 1%, or why I can't seem to get it back up to 100%, and  honestly I am not obsessing about it. Whether your happiness is at 99% or 100%, you stay in the 20% tax bracket. So don't freak out about 1%. 

the game gives you more pop up rewards like dozer parts and storage parts when your happiness is at !00%.  I've noticed.  Everyday I have a road that needs upgrading so I can never stay at 100% more than 24 hours.   Sometimes a building will look like it's covered by a park but it's really not, so add more parks or move some buildings around....that was my issue

Check your parks...its always that one that slips through the cracks that lowers that point.