Usually hard starting is one of three issues.
1. Low compression
2. Loss of prime, means air gets into the system and makes one hard to start, I had an air leak in a filter bleed screw from its O ring once, engine
was always hard to start, and sometime would shut down right after starting, requiring a restart. Eventually got so bad it wouldn't start and I had to find and fix it, it would leak air in when engine
was off, but not enough to cause running issues once running.
3. starter not spinning it fast enough, sometimes this can even be wrong grade of engine oil
. Low batteries, loose connections, worn starter.
Might want to google
Diesel loss of prime, its pretty common