Without a concrete example of a notebook showing how the data was Exported and later Imported and the contents of the text file there is some guesswork in suggesting this.

Try evaluating FullForm[yourImportedData] and I suspect you will see the numbers you exported have been turned into strings after import. This can be because of Mathematica interpreting a .txt file as containing strings rather than numbers or it can be because of the way you are asking Import to interpret the data.

If it is all strings then you might try ToExpression[yourImporteData] and see if that is enough to turn it back into numbers.