Sounds like the concrete is breaking up possibly due to age/moisture/poor mix or possibly all three!
I deally you would pull all the blocks up, clean the floor and ascertain why the cement has broken up. You also need to confirm if the floor has a Damp Course in good condition. If this is ok, then its a case of repair mortar, screed and refit the cleaned up blocks.
If the DPM is damaged or does not exist, the concrete needs repairing, then a liquid DPM laid, followed by a screed and the blocks can be cleaned up and re-fitted.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!