But those who stumbled are armed with strength (1 Samuel 2:4b NIV).
Stumbling can occur through external sources. For example, you can stumble when you hit your foot against a stone. Stumbling can also occur due to internal weakness. For example, when some muscles or tendons get sprained you can stumble.
In the life of faith also stumbling occurs. It can occur due to observing other people live in sin and disobedience. Or it can occur due to your own lack of discipline in spiritual matters.
Stumbling can occur to the strong and it can occur to the weak. It can happen when you are well and it can happen when you are ill. If you rejoice at the sight of someone else stumbling; then before you know it, you will also stumble in the same way. And stumbling has the ability to humble you of your pride.
But God does not want you to remain in the place and position of your stumbling. Even when your stumbling has been caused because of the long delay in getting your prayer answered; know that God can change the situation for you anytime. This is the essence of the thought that those who stumbled are armed with strength.
God supplies strength to His children who stumble. He provides it by coming to their aid in their time of need. Difficulties and disappointments do wear out a child of God who stumbles especially when others provoke you and stab you with words due to the lack of fruitfulness in life. But God arms you with strength and gives you fruitfulness in your life of faith!