Dull solder joint points to a few possibilities, most likely:
1. Not enough flux
2. Flux not sufficiently cleaned after reflow, causing oxidation.
Both will cause loose/weak solder joints. All you have to do is dab rosin flux on existing solder joint and apply heat with solder iron. You will see the solder joints turn shiny, immediately remove heat. Be sure to remove remaining flux with alcohol. Rosin flux won't cause oxidation as much as water-based flux, but it's acidic nonetheless.