finished and iced gingerbread biscuits WERE hard now have gone soft! why??

HI my finished biscuits were hard - perfect - then I iced them and all the ones sitting out drying AND those in airtight container have all gone really soft. is it humidity in the air?? I thought they needed to be sealed straight away to keep hard but it seems not…? I'm confused! I have sealed them up with absorbent paper but that's not working . Is it best to store them in airtight container or leave out to air??

thanks very much I'm a bit desperate!