Being a natural product, wooden stretcher bars can warp over time. This is because the wood contains different densities of material that dry at different rates. Stretcher bars are kiln-dried to prevent additional drying but occasionally this is not enough.
The best solution is to remove the artwork from the stretcher bar and re-stretch on a new stretcher bar frame.
Rather than use single pieces of wood, look out for laminated and finger jointed stretcher bars (shown below) as these are made with lots of smaller pieces of wood designed to resist all warping. Or use aluminium bars that are guaranteed not to warp at all.