Keep it simple
Compression, spark, and fuel
ENOUGH compression to start. Consider a compression / leakdown check. Maybe there is something wrong in the valvetrain, AKA not enough lash, out of time cam
Spark, a good spark and in time. You can check timing on the starter. Check the plugs for wet/ dry/ fouled. Check the spark with a solid core wire. When cranked WITH THE KEY the coil should produce at least a 3/8" nice hot blue spark
Fuel, fresh. Don't take it for granted. Easy. take an empty oil "can" poke a hole in the screw on lid, and fill half? way with fuel. BANG!!! The residual oil and fuel makes the perfect priming mixture
And........you have a good battery? Is it charged? Does the starter spin it over briskly? Does it "sound" like it has compression, or sound as if it's just free spinning?