How long have you had it? What kind of eliquid flavors are you using? Some eliquid flavors such as citrus, cinnamon, etc. are known as "tank crackers" because they are known to damage plastic tanks such as the Evod. There is a list of such flavors here on ECF, and some eliquid vendors indicate on their sites which of their flavors can potentially damage plastic tanks. Glass tanks are recommended for use with these types of flavors.
My daughter and I use Evods and really enjoy them. I hope you solve your problem to your satisfaction soon.