The problem most likely is due to your data source having different data types in the same column.
By default, Word (2002 & later) uses the OLE DB provider to get merge data from an Excel workbook. The older ODBC driver works in a similar way. Both drivers are designed to return data in a way that is compatible with Access and other relational database packages. Access requires a specific data type for each field, and every value in that field must have only that data type. In Excel, however, cells in a column (field) can have different data types. When the OLE DB provider gets data from an Excel column with mixed data types, it has to determine the data type for each column. The first 8 records are used for that (the 8 can be changed in the Windows Registry, but it’s not advisable to do so). Records that don’t conform to the determined data type are liable to not be handled correctly. To complicate matters, for historical reasons, there are two text data types: "text" (up to 255 characters) and "memo" (can be longer than that).
Some common mailmerge issues arising out of this include:
• Numbers but not text or dates being output for some records; and
• Text data being truncated at 255 characters.
Ideally, one would ensure each field has only one data type. Workarounds include:
• Inserting a dummy first record containing data in the format that is not being output correctly; or
• Reordering the data so the first record has content in the format that is not otherwise being output correctly.