My guess is that transplanting shock set them back some. Corn doesn't like to be moved.

would side dressing them with nitrogen help boost them up?

Yes, apply some nitrogen. Corn needs a lot especially in wet weather.

Since it is the lower leaves that have yellowed, to me that says transplant shock. Nitrogen can help the growth if it was not given any earlier.

I applied blood meal to the center's of the rows in a furrow yesterday morning and not an hour later we got pummeled with heavy rain all day did that just wash away all my fertilizer?

I feel your pain man!
{{gwi:73751}}

I hope my dose of nitrogen wakes them back up.