On the MAS, there is a tool that you can look at the will show if your mailbox is present. Go to C:\Avaya_Support\Tools\FEDBQueryGUI
Run this and you can choose to search by mailbox number and see if your mailbox exists.
How many MAS do you have in your MM? If more than one, do you get to your mailbox sometimes but not always? Could be that one FEDB was updated but not the other.
When you deleted it, did you validate that the 1027 updated the MAS before you readded it?
If you try the above two action steps and it is still an issue, place a call to your MM maintenance group. They may be able to simply rebuild the FEDB on the MAS or have to repair the links.
Hope this helps.