Lpc2000

hello!
I'm trying to make a routine to take care of the I2C protocol and
communicate with an external component wich is an I/O port (PCA9555
from phillips).
The problem is the interruption! When we SET the STA bit, after making
the configuration to Master Mode, the start bit is sent, the SI bit
becomes one, and now the interrupt routine should be called, and it
isn't!!! even when i use the phillips examples! the configuration of
the interrupt's is correct i suppose, once it's a copy from those
examples.
If someone could "give a hand"  with this... Thanks!