Render from fbo texture to another within same fbo - c++

Question

I'm trying to set up deferred rendering, and have successfully managed to output data to the varying gbuffer textures (position, normal, albedo, specular).
I am now attempting to sample from the albedo texture to a 5th colour attachment in the same fbo (for the purposes of further potential post process sampling), by rendering a full screen quad with simple texture coordinates.
I have checked that the vertex/texcoord data is good via Nsight, and confirmed that the shader can "see" the texture to sample from, but all that I see in the target texture is the clear colour when I examine it in the Nsight debugger.
At the moment, the shader is basically just a simple pass through shader:
vertex shader:
#version 430
in vec3 MSVertex;
in vec2 MSTexCoord;
out xferBlock
{
vec3 VSVertex;
vec2 VSTexCoord;
} outdata;
void main()
{
outdata.VSVertex = MSVertex;
outdata.VSTexCoord = MSTexCoord;
gl_Position = vec4(MSVertex,1.0);
}
fragment shader:
#version 430
layout (location = 0) uniform sampler2D colourMap;
layout (location = 0) out vec4 colour;
in xferBlock
{
vec3 VSVertex;
vec2 VSTexCoord;
} indata;
void main()
{
colour = texture(colourMap, indata.VSTexCoord).rgba;
}
As you can see, there is nothing fancy about the shader.
The gl code is as follows:
//bind frame buffer for writing to texture #5
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glDrawBuffer(GL_COLOR_ATTACHMENT4); //5 textures total
glClear(GL_COLOR_BUFFER_BIT);
//activate shader
glUseProgram(second_stage_program);
//bind texture
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, fbo_buffers[2]); //third attachment: albedo
//bind and draw fs quad (two array buffers: vertices, and texture coordinates)
glBindVertexArray(quad_vao);
glDrawArrays(GL_TRIANGLES,0,6);
I'm trying to work out what is preventing rendering to the texture. I'm using a core context with OpenGL v4.3.
I've tried outputting a single white colour for all fragments, using the texture coordinates to generate a colour colour = vec4(indata.VSTexCoord, 1.0, 1.0); and sampling the texture itself, as you see in the shader code, but nothing changes the resultant texture, which just shows the clear colour.
What am I doing wrong?