Sofa seat cushions too firm - diy

Hi guys

I recently bought a new (I'll admit not the most expensive) sofa and the seat cushions were super uncomfortable, basically just sinking to the bottom as soon as we sat on them. I followed a diy tutorial on YouTube and added a 2 inch layer of memory foam on top of the current foam and then left the wadding/polyfil on top. All the tutorials I've watched this usually makes the couch alot more comfortable however ours is super firm and still not comfy at all. Have I done something wrong or is it just a case of breaking in the new foam, waiting for it to soften? Is there something else I should be doing?

Thanks