I'm going to guess you are talking about a Cyclone Boys G4. The center pieces don't lock on to the core? I've had an issue like that. There is supposed to be a squarish stem on the core that the center piece fits onto. You might be able to, with all the outer pieces taken off, carefully turn the center pieces until they snap in. Otherwise it might be that you have stripped either the core or the center pieces, in which case I think you'll have to replace whatever parts are broken, and it might just be a good thing that you're using a relatively cheap 4x4.