Hi,
If I understood your problem then earlier you had for example 10 fields in the form promoted to the library, now you have added one more and promoted to the same sharepoint library.
If yes, then data should stay for rest of the fields but new field will get added in the default view of the library with no data at all for the existing library form items.
Now, if you will create a new item then you should be able to see that newly promoted field.
Or it can be browser issue. Try closing all the browser, clearing browser history, temp files and open browser again.
If that doesnt work too, then open the infopath form and publish it again with no changes at all.
Either of them should fix your issue.
Hope this helps!