Lost memo data in update SQL

I am using an SQL statement to update a key field in a table which contains
a memo field. When I run the query,
the memo data disappears.  Why is this happening?

Here's the SQL

         Update  ":AliasName:Levels.db" Levels
         Set Levels."Quesnum" =  "   5"
         Where Levels."Quesnum" = "  10"

The record structure is:

     Quesnum   String(5)      Primary Key
     Part           String(1)      Primary Key
     Response   String(2)      Primary Key
     Descript     String(255)
     Notes         Memo(1)

Does anyone have any idea?   Thanks in advance.

Henning von Schmeling.