If a column contains text and number, it will always be imported as text, because there is no other way to maintain as much of the content as possible. Columns containing only numbers should result in a numeric variable. Problems could be caused by multi-row headers and other fancy stuff that should not exists in data-sources.
Afaik libname uses the same mechanics, so it won't help, most likely.
Can you show the code used to read the file? Are there any interesting notes/warnings in the log?
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