Difference between writing #import<filename> in _prefix.pch and in .h or .m of a particular class?

Question

I had imported a custom Framework in my application.If I import any class of this framework in _prefix.pch ,then changes I do in the custom framework later,are not reflected in my app.But if I do the same import in the .h of any class,the the changes are reflected? I could not understand why?