I'm not a fly sparger, so forgive the dumb questions, but how much slower is your average sparge? Also, how do you decide when your sparge is done? Just the runoff volume, or the SG of the runoff at the end of the sparge?
I think rebuiltcellars is on the right track...seems like it would have to be the crush or the grains themselves.
The other thing I'd add (and I'd have to think a little more to determine if there's anything to this idea) it seems at some point the valve itself will be the limiting factor in the runoff rate. Any chance something physically loosened up or otherwise changed in your valve that increased the flow rate for roughly the same position? If so, guess that implies your runoff was slower than it needed to be in your previous batches.
Just thinking out loud.