roaming profile problem (windows server 2003)

there is a function in windows server 2003 call Active Directory Users and Computers, this function allow us to do the roaming profile for all the connected PC.
OK my question is, why some of the PCs can not upload the user profile daily? is it actually user profile will only upload once a time for a few day? or it suppose to be upload the profile each time the PC shut down? if yes, then what was happen to those PCs here??? any advise here?

thanks!