have U replaced any of them ? have U done swirl flaps?
If no , take of manifold , check each plug individually ,and if any of them faulty, replace all of them . if they all good, but u have faults that some of them faulty, U need glow plug module . Remove swirl flaps, fit blanking plates, and fits all new inlet manifold gaskets, at same time will clean its EGR valve and might need to replace vacuum hoses .do all in one go so don't need to be back few times