I dunno without more info - are you meaning Zantedeschia of some kind when you say arum lily? Growing in a pot, indoors or outside, or planted in soil in the garden? and how long have you had the plant?

i agree with Bamboo there, we need a bit more info or a photo would help.. but rough guide.... could be a young plant or if mature it could be positioning or weather related..

They take a year or two to settle in I found. Mine grow in semi shade and need a lot of water.