I've done a little experimenting, and you might try it too. Add about 5-10% of your REGULAR espresso in with the decaf, and see how the shot pulls!
I have this whole big theory about decaf coffee, and that the cell structure is compromised so much that the grinds don't provide as much flow-restriction as 'regular' would, so adding a little of the non-decaf (heh) espresso in might serve as a bit of a 'binder' that would then help the flow, and let you coarsen up your grind a bit.
Just a theory. Haven't had decaf around to play with in a while.