The causes are reasonably obvious:- These glass clamps are perfectly suitable for placing on a pipe, for which they are also widely used. However, because it is not used here on a tube, the glass clamp only presses on the polyester with two relatively small surfaces (see left photo below).- These glass clamps are also intended for straight pipes. The polyester on which they are mounted is not straight, but a curve. As a result, the mounting bracket carries all the force on two small surfaces (see right photo below).