In 1.20.1, on the latest version of the mod, the trees stop growing after a few pieces of bone meal. Even burgeoning potions don't work.

Factors that affect tree growth are biome and season (if there is a season mod enabled).
For example, a spruce tree will barely grow in a desert biome. At the same time, all trees stop growing during winter and continue again during summer. Make sure you are not being affected by these factors.