Someone mentioned the battery- Specifically, the battery and wiring
look closely at the wiring from the battery to the power side of the starter (that's the big terminal) Also check where the negative cable connects to the engine (ground).
Check every connection. Look for corrosion on the terminals. Peel back the insulation and check for corrosion (green)
Your electrical system can work fine in all other respects but if there is corrosion on those big wires it creates resistance which turns current to heat.
That will cause your starter to turn slower and fail to start the engine.
Electrical problems cause confusion. A good battery and wires will allow the engine to spin fast enough to start and that may overcome worn valves or injectors. Likewise, fixing bad valves or injectors may allow the engine to start easier despite a week electrical system.
But wires are cheap and no skill is required and it's pretty hard to make it worse so it's the best place to start.
In over my head?
First Boat, a well-equipped 1977 Camper Nicholson 31
What is all this stuff and how does it work?