Cat missing a nutrient?

My Norwegian Forest Cat loves to eat plastic bags. I've taken them away on her, tried waving my arms and yelling, I tried the 'doggie trick' of shaking a coke can with pennies in it. Nothing has even phased her, or gotten her to stop. I'll line our garbage bag with lemon juice, and she'll lick it off or rub it off until it's good enough to start munching on. What can I do to get her to stop this, and to boot, is her doing it telling me she's missing something in her diet? If so, what vitamin is plastic?