Here is a good source for facts and stats. The data is old, but gives one a general idea on the state of Orthodoxy and Christianity.
https://stewardshipcalling.com/stats-and-facts/
Except for Mormons, Jehovah's, 7th Day Adventists, and Assemblies of God all denominations are declining.
My gut feeling as to why we aren't growing are several:
1. We are losing the youth by not defending the faith against the secular world. Our beliefs are under constant attack and if leaders continue to give in (e.g. evolution vs. creationism, Gen 1 -11) and keep putting God aside then the youth will question everything and walk away.
2. Many Orthodox churches are more concerned with building grand churches instead of the spirituality of her people.
3. Youth programs are focused on non-spiritual things: dancing, basketball, meetups.
4. Youth sporting events are on Sundays.
5. Parents are not setting good examples by not making Christ the center of their home. Once they leave Divine Liturgy, it's back to "normal" for the next six days. Absent of reading scriptures, teaching the importance of prayer, fasting, alms giving.
Our church was growing prior to the pandemic. Once restrictions were lifted we have seen our numbers grow steadily. Our growth is from converts who are dissatisfied with the other denominations.