choplist & email never received

I run a couple of smallish email lists using smartlist hosted at a commercial site and recently there 
are many instances where subscribers of one list are not getting all the messages posted to the list 
(the control files for the two lists are identical). The messages are received and, according to the log, 
are properly processed (IOW, there is no difference in the log abstract between messages received 
and missing messages). I seems as if a complete address block fails to get the messages, for 
example, I have about 4 addresses at my own domain (ellerbach) that are recipients (including the 
list's digest) and if a post is missing from one it is missing from all.

Has anyone run into this kind of problem? Would changes to the choplist parameters be worthwhile 
testing (I'm using the defaults, 32 8 64 0 200000 4)?

Rich