Odd iptables issue

I know what must be the problem, but I can't find the source of the 
failure.  On the poptop server, I've got iptables set basically wide 
open now, but tried more restricted before.  I can see data going from 
ppp0 through eth1 to the work network hosts, and them replying ONLY when 
I look at eth1.  But on the same host, ppp0 still shows no reply 
packets... its like the data goes from vpn link, through ppp0 thru eth1 
to that lan, host replies, gets thru eth1, and then the iptables rules 
drop it when it tries to go back to ppp0... Why?  I'm sure there's soem 
simple rule I'm missing, but I can't for the life of me figure it out.  
I've even gone so far as to do:

iptables -I INPUT -i ppp+ -j ACCEPT

just to see if I could get it to work... and it doesn't.  I've tried:

iptables -I OUTPUT -o ppp+ -j ACCEPT

to no avail.

Ideas?

~Phil


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