LWJGL Vertex Index Buffer causes half of “Mesh” to load

Question

I'm following the 3D Game Development with LWJGL book from Antonio Bejarano. I have reached the point of being able to render a quad to the screen. After having implemented the vertex index buffer, only one triangle of the quad renders to the screen, so I think it is safe to assume the problem is with that code.

        // Vertex Index Buffer
        idxVboId = glGenBuffers();
        vertsIdxBuffer = MemoryUtil.memAllocInt(indices.length);
        vertsIdxBuffer.put(indices).flip();
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,idxVboId);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER,vertsIdxBuffer,GL_STATIC_DRAW);

The code for the vertices buffer and the colour buffer works, as I have removed the references to the index buffer and the full quad appears. I have also changed the render method to account for the new type to be rendered:

    shaderProgram.bind();

        //bind to vao
        glBindVertexArray(mesh.getVaoId());
        //enable positions buffer
        glEnableVertexAttribArray(0);
        //enable colour buffer
        glEnableVertexAttribArray(1);

        //draw verts
        glDrawElements(GL_TRIANGLES, 
        mesh.getVertexCount(),GL_UNSIGNED_INT,0);

        //restore state (?)
        glDisableVertexAttribArray(0);
        glDisableVertexAttribArray(1);
        glBindVertexArray(0);

        shaderProgram.unbind();

Any suggestions are appreciated as I have been staring at this for hours and can't really see where I've made the mistake.

EDIT: Quad Mesh Code

     renderer.init();

    float[] positions = new float[]{
            -0.5f,  0.5f, 0.0f,
            -0.5f, -0.5f, 0.0f,
            0.5f,  0.5f, 0.0f,
            0.5f,  0.5f, 0.0f,
            -0.5f, -0.5f, 0.0f,
            0.5f, -0.5f, 0.0f,
    };

    float[] colours = new float[]{0.5f,0f,0f,0f,0.5f,0f,0f,0f,0.5f,0f,0.5f,0.5f};

    int[] indices = new int[]{
            0, 1, 3, 3, 1, 2,
    };

    mesh = new BMesh(positions,colours,indices);

2ND Edit Before and after positions array change

Before

After

Answer 1

After looking through the quad mesh initialization code, it seems the error is due to vertices 3 and 2 on the second quad being the same vertex, ie both are at (0.5, 0.5, 0.0) . Perhaps you mistakenly copied the 4th and 5th vertices in your array?

ie perhaps you meant to have

float[] positions = new float[]{
        -0.5f,  0.5f, 0.0f,
        -0.5f, -0.5f, 0.0f,
        0.5f,  0.5f, 0.0f,
        0.5f, -0.5f, 0.0f
};

EDIT: Your indices seem to be wrong. Change them to { 0, 1, 3, 0, 3, 2 }