Because the only thing that is different between bumping and using starter motor, that I can see, is is the speed the engine gets turned I would expect a lazy starter. It may not be spinning the motor quick enough for it to catch on. Just a guess but it does make sense. Alternatively it might be a possible ign swith fault in that it may be intermitantly cutting off the *"ignition" as the starter motor spins.

*by ignition I mean what ever device stops the diesel reaching to the engine/pump/injectors - sorry my knowlede is lacking with modern diesels.