I recently installed a new weatherstrip on the bottom of my entry door and it appears to be too tight and could be more of a problem in the future if the wood swells. I have tried to screw the threshold down and it appears that I have it tightened all the way down, but still seems to be a little too tight. I assume it is "righty tighty", "lefty loosey". Is that correct? I looked at the packages of thresholds in Lowe's and it appears that that is the direction I should be going. Any ideas?

Righty is tighty but your threshold may already be all the way down or the cap screws are so rusted you can't turn them. Try a different weather strip, much easier than repairing or replacing a threshold.

You could try to plane a little off the bottom of the door or use a skil saw with a fence. But only cut an 1/8 of an inch off at a time until it will close.