I haven't had any problems with mine, but did ask Head for their stringing instruction which haven't been put up on the web yet. They do acknowledge that the grommet may lift so here is their advice for the mains:
"Start at the top with the center holes L34 + R34. For the left and the right side of the main strings clamp 3,25m (10,75ft) each at hole L34 + R34 respectively. Tighten 2 main strings left, two main strings right and so on. Tie off at holes L7 + R7. For best results, while tensioning the outer main strings, ensure that the grommet barrels are not lifted out of position. If barrels lift, then please push these back into position before continuing."