DIAGNOSIS The hose end has a plastic cuff that clicks onto a port on the vacuum's back. If the cuff's locking tabs are cracked or worn, or if the port is damaged, it won't stay seated.
FIX Inspect the end of the hose that plugs into the vacuum. You'll see two small plastic tabs. Make sure they aren't broken off. Then, check the silver port on the back of the vacuum body. Look for any cracks or debris preventing a deep connection. Clean the port out and ensure the hose clicks in with a positive snap, not just a gentle push.
IF THAT DIDN'T WORK If the tabs look fine, the issue might be inside the hose cuff. Sometimes the internal spring mechanism that holds the tabs out gets jammed. Try working the cuff back and forth and spraying a little compressed air around the moving parts to dislodge any grit.
HEADS UP If you need to replace the hose, don't just buy any Shark hose. You need the specific model for the NV352, as the connectors differ between generations. The part number is usually on a sticker on the hose itself.