I'm going to take a guess without knowing anything about your set up (other than you said that you just started it) and say that it's because your system is new, and corn is the Grandaddy of all heavy feeding plants. Plants like that (super heavy feeders) generally seem to do much better in an AP system once the system has been running a good while (6 months to a year) and has built up a decent store of nutrients.
First off I agree with Vlad's comment. Secondly, most grains (esp corn) does not like wet feet and would do better in a flood and drain system so I wold say that your plants are either malnourished or suffocating.If the roots look healthy, you might try peeing into your system ...or add hydroponic nutes if you don't have any/ many fish.
I echo the last two comments. Corn is a heavy feeder and requires a ton of nitrogen.
I've noticed that in NFT or Raft culture, anything that limits certain nutrients seems to show deficiencies far more in those methods than in Media beds.
4" pipes are not where I would grow corn and I will predict that roots will be clogging things on you fairly quickly, keep an eye out for when you need to start root pruning to keep from clogging/overflowing your pipes.
nft and corn=bad mojo.
corn in media=better mojo
if leafs are molted , dark green vains and yellow on the outside it could be iron deficiant.