JavaFX populate TableView from ObservableList

I'm trying to populate a TableView with data from an ObservableList. I've done this before but for some reason I can't get it to work now. I don't get any exceptions or anything but it simply doesn't add anything to the table.

this question is similar but I identified another issue caused by JPA which made it so the mentioned constructor for Modification is never executed. Instead JPA does its magic to assign the values.

Here's my code - I snipped code that was irrelevant to the question:

FXML

<TableView fx:id="tblModifications" layoutX="14.0" layoutY="14.0" prefHeight="545.0" prefWidth="512.0">
    <columns>
        <TableColumn prefWidth="75.0" text="%itemNumber" fx:id="colModArt"/>
        <TableColumn prefWidth="75.0" text="%name" fx:id="colModName"/>
        <TableColumn prefWidth="75.0" text="%amount" fx:id="colModAmount" />
    </columns>
</TableView>

Main.java

public class Main extends Application implements Initializable {
    private TableView<Modification> tblModifications;
    private TableColumn<ObservableList<Modification>, String> colModArt;
    private TableColumn<ObservableList<Modification>, String> colModName;
    private TableColumn<ObservableList<Modification>, Integer> colModAmount;

    @Override
    public void initialize(URL arg0, ResourceBundle arg1){
        colModArt.setCellValueFactory(new PropertyValueFactory<>("barcodeProperty"));
        colModName.setCellValueFactory(new PropertyValueFactory<>("nameProperty"));
        colModAmount.setCellValueFactory(new PropertyValueFactory<>("amountProperty"));

        admin = Administration.getInstance();

        tblModifications.setItems(admin.observableModifications); // This list is populated with correct data, I tested.
    }

    @Override
    public void start(Stage primaryStage) throws Exception{
        resources = ResourceBundle.getBundle("strings", new Locale("NL"));
        Parent root = FXMLLoader.load(getClass().getClassLoader().getResource("main.fxml"), resources);
        primaryStage.setTitle(resources.getString("title"));
        primaryStage.setScene(new Scene(root, 1280, 1024));
        primaryStage.show();
    }
}

Modification.java

public class Modification {
    public SimpleStringProperty barcodeProperty;
    public SimpleStringProperty nameProperty;
    public SimpleIntegerProperty amountProperty;
    private int id;
    private Seller seller;
    private Product product;
    private int amount;
    private Boolean accepted;

    public Modification(int id, Seller seller, Product product, int amount) {
        this.id = id;
        this.seller = seller;
        this.product = product;
        this.amount = amount;
        this.accepted = false;

        barcodeProperty.set(String.valueOf(product.getBarcode()));
        nameProperty.set(product.getName());
        amountProperty.set(amount);
    }
}

Any help in resolving this issue would be greatly appreciated!