You might be missing the little spring just on top of the selector plate
Like the one in this picture:
Without that spring, your selector plate will make your rifle fire full auto regardless of the selector switch position, because that spring make the semi/auto latch (for a lack of proper name) move to semi-automatique in the mechbox (someone else might be able to explain that mechanisme better than me).
Anyways, a spring from a bic lighter, or from an old hop-up, cut to proper lenght will do the trick.