If you're having an issue with this still i found a fix. Change your mode from Auto to manual in the day/night button in the menu tab. Make sure all mods are off and check the items in front of your stairs. Go into your storage and sell all the item types you have in your storage that are in front of your stairs. Don't worry because they'll be easy to get back. Now you'd think you're freight workers would grab the stuff, but no. Now you have to switch to exploration mode because only your adventurers will grab this stuff. You might have to exercise patience here as there might be some freezing and lagging while it waits for an adventurer to grab the stuff, but as soon as they do the game should speed back up. Things to note, while the adventurer is stuffing stuff in the storage it is still filling so keep selling the accumulated items. Also try not to have more than twelve adventurers at a time. Recruit new adventurers to unlock new recruits and items but always dismiss them before beginning a new exploration. Lastly take advantage of manual mode it can really help you set up your storages if the item mod keeps getting too stacked. Hope this helps