Use a copper braid and soldering iron to remove all of the solder from the contacts and holes. It is soooo important to remove ALL solder before proceeding. If you are having trouble removing solder, clean with 99% isopropyl alcohol, reapply flux and try again. Sometimes it is helpful to ADD more low heat solder and then remove it.

After you remove the solder you will be left with a brownish gunk/sludge from caramelized flux. This needs to be removed. Use 99% isopropyl and a qtip. When you are finished your board should look like the one in the second picture and you should be able to see clearly right through the holes.