Issues with lightmapping.

I am seriously getting frustrated trying to lightmap my scene. It's just not working! Everything is set to static, and all the models have been lightmapped before but it's just in this scene where things don't feel like working. Is it because the terrain is to big?

My biggest problem is just that the trees on the terrain are not being lightmapped, and after lightmapping when I choose "Baked shadows only" the whole terrain pretty much turns black. The shadows that DO appear end up splotchy. This is driving me insane. I've lightmapped before and it's worked fine, but i just don't understand what isn't working this time.

here's what I mean:
http://imgur.com/a/S0aGW

Also it might be important to note that when lightmapping, the little blue bar in the bottom right corner just quits before halfway, it never makes it to the end. No errors are shown or anything though. Can anyone help me out please? I'm going crazy.