Check engine compression. If the compression is too low, the engine won’t start. This can be due to a variety of reasons such as valve clearance being out of adjustment, worn valve guides, incorrect valve timing, or worn piston rings. You can perform a compression test to see if this is the problem. If the compression is low, you’ll need to inspect and potentially repair these components. It might be a bit more technical, but it’s worth checking if the other suggestions don’t solve the issue. Ensure the battery is fully charged before conducting a compression test.