I had this problem with my Vader II lightsaber and contacted Diego at Attakus for some advice on correcting it and he was kind enough to send me a replacement free of charge. Great customer service.
Unfortunately the strength of the plastic used for the lightsaber blades isn't strong enough to support the weight of the blade itself. And because the lightsaber's position is angled instead of vertical gravity will eventually force it to bend. One way you can prevent this is to rotate the blade every now and then. What I mean by that is to turn the blade itself inside the saber's hilt so the weight of the blade isn't always putting pressure on the same point. That way if it starts to bend one way eventually it will start bending the other way once rotated and correct itself. The trick is to do this often enough that the blade never gets a chance to bend. I've been doing this every couple of months with my new Vader II blade as well as Yoda's and its been keeping them straight. Hope that helps.
I'm pretty sure this problem has been corrected on new releases though. I've had my General Grievous around a year now and thankfully they've never shown any signs of bending.