I gather that the destination field is text? That would mean the issue lies at the source or occurs in the import/transport. How you are moving the data might help others help you (transfers are not my strong point). Also might help to know what the data type of the source field is - or are you melding from different fields/columns and the source is text but is formatted as a number? Sometimes it helps to import into a staging table in Access, then use update/append queries to move the data into another table whose fields are properly designed. Then numbers as text can become actual numbers.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.