There's a few possible answers to your question :
1) Use the manufacturer supplied charger for your phone. Avoid charging via USB to your computer. It's not harmful, but, a much slower charging cycle vs. a wall A/C outlet.
2) You have a worn battery or loose-fitting charging cable.
3) Some of the newer phones actually charge quite a bit slower that others. This is an intentional design by the manufacturer to keep the battery cooler during charging, which prolongs overall battery life.