I don't think you have to look much further than the logistical issues you already mentioned, just dig deeper into them.  While I know there are other signifigant ports, NOLA is the big player.  The corn crop in the south got gobbled up super early last fall, so none of it is left to export.  The corn belt was pretty nearly sucked dry of corn after the 2012 drought, so the corn that got sold off the combine pretty much stayed close by to recharge domestic users, while soybeans went down the river.  The way things look so far it will be a miracle if any of the places north of St. Louis that have surplus corn can get it down the river for 3 more weeks.  Heck, they were hoping to open the river March 15 here but every night in the 10 day forecast is single digits or below zero right now.  The rail bottlenecks have been pretty well known all winter.  Let's see how the March 23 and March 30 corn export inspection reports turn out when those port loaders actually get some supply in their hands.