During strong, sunny conditions the amount of light and the resulting high temperatures under a cover may become critical, causing young plants to wilt or even die. In these circumstances, you should shade the plants.
Glass covers can be shaded by applying a milky shading wash or place shade netting or horticultural fleece over the glass. You can also shade plastic propagator lids by draping the same materials over them. When using polythene bags, swop the usual translucent ones for milky ones.