lightning network is not exactly something that is implemented one day. it is like a protocol which is already there and now there are applications being built based on it.
it is like asking when will bitcoin be implemented! bitcoin protocol is already there, and many programs are built to use it such as bitcoin core, electrum,....
I wonder when this L network can massively be adopted to give ease to its users? And what are the problems hindering these changes?
I would say ease of use and stability mostly.
Ease of use because this whole concept of payment channels is still fairly new and developers need to find a proper way to package the details of opening / closing a channel into a practicable user experience.
Stability in that there's probably still a lot of edge cases out there that need some testing to be done, otherwise we'd have people lose their coins on a regular basis. Not irrecoverably lost, mind you, but likely in a way that would require a multitude of technical steps that you can't expect from the average user. Even without people losing coins there's still aspects such as routing and connectivity to which a lack of stability is detrimental.