ADVENTURER LIMITATIONS
Adventurers are currently limited from growing progression by 3 primary mechanisms

1 - Weekly Tome Caps
Adventurers are limited to obtaining a specific amount of tomes each week which in turn limits the amount of new top tier gear they can purchase to improve their characters. As an example, they are limited to 450 Poetics Tomes each week right now.

These items are not as good as items from #3, but can be as good or better than #2

2 - Weekly Drops
Adventures can only receive 1 drop per week from an instance that is repeatable that week. A great example of this was Syrcus Tower before the most recent patch.

These items are not as good as #3, are usually equal to or better than #1, but can be worse than #1 on occasion.

3 - Weekly Attempts
Adventurers can only run and complete and attempt to get a drop from the highest end instances once per week. Currently this applies to Coil.