question regarding new email not delivered but replies are

Hi everyone,
 
I have to admit that I am not a majordomo user.  I'm a list member of a MD
list.  There is a continuous issue with the list and the owner can't seem to
figure out what the issue is.  Since I'm fairly technically competent I was
hoping to possibly figure it out and pass the information on.  
 
The issue is that many of the list members cannot post email to the list
UNLESS it is in reply of someone elses post that has already been delivered
to the list and even then it does not always work.  It is about 90%
consistent with the specific users that are having the problem.  We can find
no common denominator between these members.  They are using pcs and macs,
different email programs and different isps.  
 
Might there be a known issue that is easily explained?
 
Thanks!
 
cheryl webber