TableView: Only some column data showing up

Question

So I'm trying to get some TableViews to work and I've managed to get some of them working but some of my columns' data is missing and I don't quite get what's the difference that make some of the columns show up and others not. Here's the code for the initialisation of the tables:

void initTables(TableView<Layer> layerTable, TableView<Section> sectionTable, TableView<Edge> edgeTable){

    // Columnas de tabla ejes
    TableColumn<Edge, Integer> indexEdgeCol = new TableColumn<>("Numero"); //Not showing up
    indexEdgeCol.setMaxWidth(80.);
    indexEdgeCol.setCellValueFactory(new PropertyValueFactory<Edge, Integer>("Id"));
    TableColumn<Edge, Double> sepEdgeCol = new TableColumn<>("Separación"); //Not showing up
    sepEdgeCol.setMinWidth(110.);
    sepEdgeCol.setCellValueFactory(new PropertyValueFactory<Edge, Double>("Separación"));
    TableColumn<Edge, Double> loadEdgeCol = new TableColumn<>("Carga");
    loadEdgeCol.setMinWidth(110.);
    loadEdgeCol.setCellValueFactory(new PropertyValueFactory<Edge, Double>("Carga"));

    edgeTable.setItems(getData().getEdges());

    // Columnas de tabla tramos
    TableColumn<Section, Integer> indexSectionCol = new TableColumn<>("Número");
    indexSectionCol.setMaxWidth(100.);
    indexSectionCol.setCellValueFactory(new PropertyValueFactory<Section, Integer>("Id"));
    TableColumn<Section, Double> lengthSectionCol = new TableColumn<>("Longitud"); //Not showing up
    lengthSectionCol.setMinWidth(120.);
    lengthSectionCol.setCellValueFactory(new PropertyValueFactory<Section, Double>("Longitud"));
    TableColumn<Section, Integer> slabSectionCol = new TableColumn<>("Num. Losas"); //Not showing up
    slabSectionCol.setMinWidth(125.);
    slabSectionCol.setCellValueFactory(new PropertyValueFactory<Section, Integer>("losas"));

    sectionTable.setItems(getData().getSections());

    // Columnas de tabla capas
    TableColumn<Layer, Integer>indexLayerCol = new TableColumn<>("ID");
    indexLayerCol.setMaxWidth(40.);
    indexLayerCol.setCellValueFactory(new PropertyValueFactory<Layer, Integer>("Id"));
    TableColumn<Layer, String>nameLayerCol = new TableColumn<>("Nombre");
    nameLayerCol.setMinWidth(130.);
    nameLayerCol.setCellValueFactory(new PropertyValueFactory<Layer, String>("Name"));
    TableColumn<Layer, Double> modYoungLayerCol = new TableColumn<>("Mod. de Young");
    modYoungLayerCol.setMinWidth(101.);
    modYoungLayerCol.setCellValueFactory(new PropertyValueFactory<Layer, Double>("Young"));
    TableColumn<Layer, Double> thickLayerCol = new TableColumn<>("Espesor");
    thickLayerCol.setMinWidth(101.);
    thickLayerCol.setCellValueFactory(new PropertyValueFactory<Layer, Double>("Espesor"));

    layerTable.setItems(getData().getLayers());

    edgeTable.getColumns().setAll(indexEdgeCol, sepEdgeCol, loadEdgeCol);
    sectionTable.getColumns().setAll(indexSectionCol, lengthSectionCol, slabSectionCol);
    layerTable.getColumns().setAll(indexLayerCol, nameLayerCol, modYoungLayerCol, thickLayerCol);
}

As far as I know, data is as it should be, it's only that it's not getting shown in said tables and I don't quite know what I'm doing wrong, so any help would be appreciated. Thanks in advance!

EDIT: As requested, here are my Section and Edge classes:

Section.java:

import java.util.ArrayList;

public class Section extends Object{

    private final int id;
    private int bloques; 
    private double length;

    public Section(int id, int bloques, double length) {
        super();
        this.id = id;
        this.bloques = bloques;
        this.length = length;
        // this.start = start;
        // end = length + start;
    }

    public int getId(){
        return id;
    }

    public int getBloques(){
        return bloques;
    }

    public void setBloques(int bloques){
        this.bloques = bloques;
    }

    public double getLength() {
            return length;
    }

    public void setLength(double longitud) {
            this.length = longitud;
    }

    @Override
    public String toString() {
        return "Section [id=" + id  + ", length=" + length + ", bloques=" + bloques + "]";
    }
}

Edge.java:

public class Edge extends Object{

    private final int index;
    private double sep;
    private double carga;

    public Edge(int index, double sep, double carga){
        this.index = index;
        this.sep = sep;
        this.carga = carga;
    }

    public int getIndex(){
        return index;
    }

    public void setSep(double s){
        sep = s;
    }

    public double getSep(){
        return sep;
    }

    public void setCarga(double c){
        carga = c;
    }

    public double getCarga(){
        return carga;
    }

    @Override
    public String toString() {
        return "Edge [index=" + index + ", separación=" + sep + ", carga=" + carga + "]";
    }
}