The reason why this happens is because of a phenomenon called age hardening. Carbon will bend and warp after being forged due to the nature of the metal. Forgers resolved this problem by adding a thin strip of carbon to a thick piece of steel. Even if a knife is purchased in good straightened condition, both metals will naturally want to bend in opposite directions over time. There are knives that were poorly made and cannot be straighten well, and there are knives that can be straightened and used without a problem. However, it is pretty impossible for a knife to be completely straight and stay straight forever. All yanagi knives need to be straighten out by a professional every now and then. Some places offer this service for free, so you could try asking whoever you purchased from. Honyaki knives are MUCH harder (and scarier) to fix, so make sure whoever you ask to fix can do it well.