Tim Chester says we can plateau for a couple of reasons: we stop fighting sin once we get rid of the obvious sins of behaviour, but never confront the more subtle sins of the heart; or we only think we've plateaued, because as we grow in godliness, the more subtle sins of the heart become clearer to us (I can testify to that!).
We might also stop growing because we're missing something. Here's Chester's summary of the steps in You Can Change, which I thought would be good revision for us all:
1. Keep returning to the cross to see your sin cancelled and to draw near to God in full assurance of welcome.
2. Keep looking to God instead of to sin for satisfaction, focusing on the four liberating truths of God's greatness, glory, goodness and grace.
3. Cut off, throw off, put off, kill off everything that might strengthen or provoke sinful desires.
4. Bring sin into the light through regular accountability to another Christian.
If you resist doing these four things, and we often resist numbers 3 and 4, it's a sign that you still treasure sin in your heart: sin is still more important to you than God. Turn to God in repentance, reflect on sin's consequences, meditate on the all-surpassing glory of Christ, and beg God to give you a love for him that eclipses your love for sin.
It's scary, isn't it, how much we resist true change! Let's keep fighting, and praying for God to help us love him more than we love sin.