Different batches of express (or any pellet for that matter) make a big difference Jimbob. I have a mega batch from 2010 and have yet to find anything better. Totally consistent and predictable.
Batches I tested from 2012 and 2013 were particularly bad in terms of producing fliers. 2012 were the worse and actually had me thinking that they were a mix of batches.
Couldn't get them to group at all.
As has been said many times before, test a batch thoroughly before you buy and when you find the right one buy as many as you can afford. It is always good to have a reference batch that you can compare with.
What the batch number of the one you are having problems with? I may have also tested it.