Fixing Sticky Seasoning on Cast-Iron Pan
You don't have to re-season from scratch, just get rid of the sticky residue and make sure that the next layer of seasoning is very fully cooked on.  The easiest way to remove the sticky residue is to scrape off what you can with a spatula, scrub off the rest with kosher salt and baking soda, and clean that up. Then give it a couple more passes of seasoning to even out the layer, making sure to leave it in the oven extra-long for the next seasoning.  The extra cook time helps avoid having a sticky spot reappear.

What causes this?
This happens when the oil pools in one spot, and doesn't get heated long/hot enough to form the seasoning layer.  If you resume seasoning without removing the sticky residue, it will form a bubble of uneven seasoning that flakes off, exposing the less seasoned stuff below. I learned that the hard way.