When a time log does not have a Project assigned, it will always be shown as non-billable until a project is selected for it. In case you are using a task-based billable rate, a task also needs to be selected for it to be billable.
If some logs on otherwise billable projects appear as non-billable, this could be due to incorrect bulk editing.
Let's say a member initially enters the time without the project selected and then goes to bulk edit the entries to select the project for multiple logs at once. In this case, the time logs would be entered as non-billable initially (because they didn't have a project selected).
If they then add details by bulk editing the time logs, they should mark them as billable in the bulk edit form. Otherwise, 'no change' is selected by default and this causes the entries to remain in the non-billable state, even when the project was selected for them in bulk edit form.
To avoid this, it might be helpful to turn on the option to 'Require project and task inputs on every log', here. Then, every added time log will need to have a project and task selected right away.
You can also disable normal users from being able to change the billable status of the time logs from billable to non-billable and vice versa. This means they will not be able to mark logs as non-billable on billable projects. Please go to the Tracking form section of the Account settings to set this.