varnish bottleneck?

[ll]

I think there are maybe some problem about varnish .My varnish's version
is 2.0.4 .I want to cache a website for everything .so I set the rules
like that

if (req.http.host ~"www.abc.cn"){
lookup;
}
it's well. Varnish can cache everything .but some function of the
website is unable .eg POST. So I put the POST judge before the HOST
.like that :
if (req.request == "POST"){
pipe;
}
if (req.http.host ~"www.abc.cn"){
lookup;
}
there are some problem .many of url's record can't be find in the
varnishlog .and there are no marked by the varnish eg "X-Cache: MISS" in
the Response Headers .
I had post this problem in this maillist .it's nothing about pipe or
pass about no marked. And there urls will go to the backend server every
time. I think is there the varnish bottleneck? whether varnish judge the
POST first ,it can't handle all .so there are some miss handle ,and go
though to the backend ? or maybe some options ,I didn't have the right
configure in the vcl ?