What's killing my privet hedge?

We have mature and maintained privet hedges on three sides of our (clay-heavy) garden. But, after about 15 years of looking after the hedge: it has started to die in different places. First an individual plant will have the leaves go light green; then turn a mottled brown as they wither; and then after about 3-4 weeks the leaves all drop off. After another 3-4 weeks, the bushes are brittle and the branches readily snap off. Then the next bush along the line goes the same way. I tried digging the dead ones out and also replacing the topsoil of the dead bushes and removing another 2-3 plants either side, then replanting with laurel (not necessarily the best substitute) but the privet continues to die, metre-by-metre. What's killing it, and what can I do??