I suspect that you have a heirarchical folder system and that there are sub folders located off different trays that use the same folder name.

2017
  |
  Accounts

2018
  |
  Accounts

You will find that copying messages into accounts will not necessarily end up in the folder you thought it would go to. You need to eliminate duplicates (even though they are child items off different parents).

To fix it you would need to rename the above to  Accounts 2017 and Accounts 2018.

I'm sure your messages are there. You just need to look in all the child folders that may have the same name.

When I drag a message from one folder to another, the message get lost. What should I do? Thanks.

What do you mean by the message gets lost?  Please explain exactly what you are doing and what you see.

Does it happen all of the time or just with specific folders? 

Is the message gone from the original folder?  

Is the destination folder open or closed when you drop to it? If closed, try with it open.  If the message gets moved but disappears after the folder is closed then opened indicates you have a folder level filter acting on it.

Do you have any autofiltering in place? 

Have you searched the entire mailbox for one of the lost messages to see where it ended up?

[quote user="Brian Fluet"]

What do you mean by the message gets lost?  Please explain exactly what you are doing and what you see.[/quote]

I drag a message from one folder to another folder.

[quote user="Brian Fluet"]Does it happen all of the time or just with specific folders? [/quote]

 All folders

[quote user="Brian Fluet"]Is the message gone from the original folder?  [/quote]

yes

[quote user="Brian Fluet"]Is the destination folder open or closed when you drop to it?[/quote]

open

[quote user="Brian Fluet"]Do you have any autofiltering in place?  [/quote]

no

[quote user="Brian Fluet"]Have you searched the entire mailbox for one of the lost messages to see where it ended up?[/quote]