Answer: I can't think of any obvious answers, but here are a few possible factors. Try using a purchased potting mix in the planter, rather than soil from your garden. Be sure there are drainage holes in the bottom, or if it is a large planter add a layer of gravel in the bottom. Plants that are waterlogged--even if they receive sun--can suffer root damage. I would use a mixture of potting soil and a light soilless peat/vermiculite mix, to help with drainage.
If the box is small, it could be that the bricks are heating up, and essentially "cooking" the roots. Have you checked the plant regularly, to see if it is drying out too much between waterings? Too much and too little water are the most common problems with container plantings.
Try planting truly heat-loving plants in that box--vincas might do well, depending on the amount of sun. Portulacas might also work. Petunias, especially the 'Wave' series, migh also do well.
Q&A Library Searching Tips