Couple of quick points, earth's have been mentioned. The main one on the chassis by the battery take this off be clean it. This will cause all manner of strange problems. You had spark but is the fuel pump running? Turn ignition on it should buzz for a few seconds before you crank it over. But if these are ok, amplifier is likely cause , breaking down gradually. I have seen a few over the years.
The fuel pump is running. But I had only one spark, per starting attempt. To my understanding and if I remember well, the spark must be continuous, as long as the engine turns.
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