Check the voltage AT THE BATTERY while the starter is turning.  Once
when I had a weak battery the starter would turn the engine over fine
but the engine would not start.  I believe the voltage AT THE BATTERY
was 10.25 volts while cranking.  The engine would start quickly when
jumped with another battery.  I ran the engine for a few minutes to
charge the battery a little and when the voltage was >10.75 volts, while
cranking, the engine would start.  I believe the voltage was high enough
to run the starter but not high enough to run the FI/Ignition digifant
system.  To check this out I ran the lights to discharge the battery and
once again, when the voltage was <10.5 volts, the engine would turn over
with the starter but would not fire.

The voltages mentioned here are my best recolection of what I measured
and they may actually all be 1 volt lower.

Try measuring the voltage and then starting with a booster battery.

Replacing the battery fixed my problem.

Of course, you may have something else wrong.

Good luck.