An ES spread with net spread b/a = 17.25/16.00. I went thru 16.25, 16.5, 17.00, still no fill. I got curious if the system is whacked so I changed the BUY LMT to 17.25 to the now 17.25/16.50 b/a, and still no fill for 5 min. I canceled the order and placed each leg separately, each filling IMMEDIATELY for a net price of 16.75. Now how is it I could buy the legs separately at 16.75 net and the spreader will not pick it up at 17.25 for 5 min??? Smart routing on Globex.