starting problems

have a 300tdi 96 auto have changed the starter motor alternator and new battery and all was fine . but now having problems starting it again seems very lazy turns over but stuggles a bit to start glow plugs fine battry fully charged alternator charging well and its a new starter motor did try earthing started with some jump leads to battery made no realy diffrence any help

As a suggestion try jump leads direct from battery to both relavent terminals on starter. If still sluggish gotta be starter or battery. If ok try one lead at time to see if fault on earth side or on live side??