The reason the veneers keep falling off, is difficult to ascertain without an individual examination of your situation. However, if you already have root canal treatment on those teeth, the more appropriate treatment would have been full coverage Crowns. The crowns can actually be made metal free, out of zirconia which is white. The best thing to do at this point is to go and see a specialist, the Prosthodontist. Just look one up in your area. Best of luck.