Many Christians never grow up (Hebrews 5:11-14).
They don't understand their faith, and they depend on themselves or secular wisdom to get through life.
Other Christians grow at first but get stuck in an adolescent stage of faith.
No matter how long we've been Christians, we need to keep growing.
If you feel you've stagnated, here's 2 Suggestions:
1. Pray for the desire to grow.
Ask God to help your "unbelief" (Mark 9:14-24) and ask Him to help you realize your desperate need for His Word. You literally cannot grow without it (Matthew 4:4, Psalm 19:7-14; Psalm 119).
2. Do something!
Commit to something, even something small (e.g. 5 minutes daily Bible reading) and watch that investment grow like the investments of the faithful servants in Matthew 25:14-28. (For additional encouragement read Baby Steps)
It isn't rocket science--it's pretty basic: growth requires planning and perseverance. We've got to realize our need and do something deliberate about it.