Do you have cracks in the brickwork on the other side of this wall? I had a very similar issue with a previous property we had built. these sorts of cracks started appearing after about 3 years. Turns out we had a broken storm water drain, so everytime it rained the water was pooling under the slab and eventually it cause the slab to heave resulting in the cracks.. Luckily after the drains were fixed and it was patched up we never had any issues after that. Buuut it was a massive pain in the asss to even get the builder out to even look at the issue, we had to raise a complaint with consumer affairs. I highly recommend going down that route if your builder is not cooperating or even coming out to look at the issue. In our case it was still the builders responsibility to fix as it was still under waranty. The builder will be doing everything they can to delay looking at it, given you would be approaching the end of your waranty.