When in doubt, lift more. The only way to increase your strength is through progressive overload. This means that as time goes on, you have to keep improving the weight that you lift in order to continue making gains. The way it works is that your muscles are like a muscle membrane. If this muscle-membrane grows beyond its previous size, then it will tear and split, allowing for growth to take place in the area that has been torn or split. In order for muscles to continue growing they must be fed the nutrients and such necessary for growth continuously; if there is a decrease in the feeding of this muscle-membrane, then there shall be atrophy (muscle wasting).