so i am running a campaign, and two of my player characters aren't leveling up. there both on the milestone leveling system. does anyone know what to do / how to fix this.

After their characters gain a level, the players must go into the builder to the Class tab and specifically add that level to their class. The system doesn’t do that automatically in case the players wish to multiclass.

its milestone, and i've told my players to set their levels to level 3, but two of my players have this bug to where i cant see them level up their character.

Hmmm. As the DM, can’t you just access the builder for their character sheets and level them up for them?

I can, but I don't know if that will work, right know I'm testing if my player can see a dagger that i placed in their inventory, but as of right now that is the best i can do to figure our what is going on.

It should work. As the DM you have total access to their characters. You should be able to do anything you want or need to do with their characters.