My Miata was only hard to start when the engine was warm and on a warm or hot day. When the engine was cold it would start fine. But then when it was hot and on a hot day that took it a long time to cool down it would just crank and not fire up.
For a long time I was only receiving an EGR error code. PO402 EGR Flow Excessive Detected. It took a period of about two years and then it started throwing out a cam position sensor code. I replaced it and both of my errors went away. Prior to that I was replacing wires and plugs each summer. That seemed to get me through. This time I replaced the coils and that did nothing to help. So even if you are not getting the cam position sensor code on your miata You might think about changing it out.