Plants need to be fully mature before they become fully productive and able to continually bear fruit in season. (Some nut trees take 10 years or so before they are fully productive.) I think that may be true for some Christians too. Until plants are fully mature they usually only produce smallish or immature fruit, or they only fruit sporadically. Some immature plants may begin to form fruit but are unable to hang on to them, so they are dropped before being fully formed.
However for some, the most spectacular growth can be seen in the first weeks after conversion, because they are changing their behaviour to be less worldy and more Christ-like. The development of a Christ-like nature is true fruitfulness.
"For the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self control. Galatians 5:22-23
Sometimes, like plants, we go through seasons. There may be times because of different circumstances that we have less opportunity to "shine our light" in the world. Having said this, the Bible tells us that we must be ready in every season to share the gospel. (2 Timothy 4:2)
We must be doers of the word and not just hearers! We must put the teaching of the Bible into practise. (Matt 7:24-27; James 2:22-25) We must be obedient to Jesus and the leading of the Holy Spirit and then we will truly bear the fruit that God wants us to bear, that is, to do the work that he has prepared for us. We will be rewarded for what we have done in obedience to Him. (Rev 22:12)
"For we are God's workmanship, created in Christ Jesus to do good works which God prepared in advance for us to do." Ephesians 2:10
"You did not choose me, but I chose you and appointed you to go and bear fruit - fruit that will last." John 15:16
Obedience to God, and continually abiding in Him, are the keys to fruitfulness. (John 15:1-8) If we do this, God will take care of the rest.
Also we must not compare ourselves with other Christians. We are all unique with different abilities, callings and "journeys" with God. God does not compare us with others - He just loves us.