How can I increase the number of stones that fall more and more?
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You will have to provide a project (.c3p - construct.net/en/make-games/manuals/construct-3/overview/saving-sharing-projects) and provide a bit more explanations in regards to what you have done in your project, where in the project are those stones and what you would expect your project to do, in regards to what you are actually experiencing it to do.
Right now, you are asking crystal ball prediction and I'm afraid no one can really answer your question in the current state of informations you provide.