hi im just begining to use the pic18 series

my current setup is:
a melabs USB programmer
PBP 2.45A
Microcode Studio 2.3.0.0

my problem is that when i try to do a really basic thing such as make portb.0 go low it doesent work properly

the following has no effect :
PORTB.0 = 0

however this works for some reson:
low PORTB.4

thanks in advance for any help and responses