Soldering issue with arduino
Fair play, that is a lot of flux residue. I am assuming you are using some quite chunky solder with lots of flux in it.

Flux is non-conductive, so the residue will be forming an insulating layer on the outside of the solder. That doesn't mean the joint is bad, only that you can't get to the joint because the flux residue is in the way.

If there is a conductive path between the tip of the pin and the pad next to it then all is fine. The fact that you can't see the signal through the outside layer of flux is neither here nor there.

If you want to clean it off you can use isopropyl alcohol (AKA PCB cleaner, AKA "rubbing alcohol") and use, for instance, a tooth brush to scrub it off.