ipn failed

my site is using website payment standard , it work well before but recently there are some order that ipn status http code 400 and its status retrying , and surely these order payment recieved and not recorded in my database, however some order i recived still fineworking and record correctly in my admin order part

i've looked into the failed ipn message id in paypal didn't found anything wrong, so what's going on , it is have something wrong with v1.39d , i have upgraded to this few weeks ago, but i don't know whether it's related~ , any possible reason to cause an ipn status retrying and its http code is 400?

also if i need to complete resolve the problem , express checkout module is the best way? do i need an business paypal a/c for this module? thanks