If you left an item on your flight you should let the staff know if you're still at the airport and have someone with the airline go back onto the plane to check to see if the items or item is still on the plane and if so they can retrieve it for you.
Let the staff know what you left behind and give them a good description of the items and your seat number.
If you've left the airport you should contact the airlines customer service number and report the lost item or items and be as detailed as you can including your name, address and seat number and if they find the item you can either pick it up or they can ship the item back to you at your cost.
The reason why you are not allowed back on a plane after getting off is mainly due to security and safety.
Getting back on a plane after getting off without going through security again compromises the security and safety of the other passengers and airline employees and flight.
If airlines allowed passengers to go back onto the plane after they got off without going through security again, it would compromise the safety and security of the flight and airport as well as efficiency.
That would mean that even though a passenger had previously passed security checks and has flown on the plane without any issues, the passenger could go back onto the plane with something dangerous such as a bomb, gun knife etc or could decide to in that time plan to hijack the plane eventually.
So airlines have to stop people from going back onto the plane to get items after they've gotten off.
After you leave the secure aircraft or plane area, you are now then outside of the monitored and secured zone and any re-entry into the secured area and plane would require that you go through security again.
Also aside from safety issues and security issues, allowing airline passengers to re-board the airplane even to look for something or retrieve something would also disrupt the airlines tightly scheduled flow of aircraft turns and the necessary procedures, like the accurate reporting of passenger and baggage counts, which is also crucial for the departure of the flight.
Aircraft turnaround is also a quick process and having any disruption can cause airline delays and allowing someone back onto the plane also disrupts the flow of other passengers, baggage and crew and makes it difficult for the airline to stay on schedule.
If you've left something on the plane you should let the airline know about it immediately.
If you're still at the airport, return to the gate or speak to an airline representative at the gate and inform the airline staff about the item you left on the plane and the location of the plane.
A staff member can then check the plane for the item or items before the airplane departs and if they find it they can give it back to you.
If you're left the airport you should contact the airport through their number or through their website or app as soon as possible and fill out a detailed lost item report.
Include the flight number, seat number, date, time and a thorough description of the item, which includes the bran, color and size.