To learn new limit breaks you have to use your current limit breaks tones of times. What you should do, is find a monster (like those wasps to the south) that casts "Fury". Allow yourself to be furied so that your limit guage rises quicker.

Then just spam Bigshot/Sledfang, W/e your using and you should get a new one.

I'm pretty sure thats how it works. Always worked for me.