Hi Bill, I hope you don't mind if I ask, What American records do you have for this couple?Do you have the death records for Agnes and Andrew? A marriage record? Naturalization papers for Andrew? Birth and death records for their children? Such records should (or might) state parents for them, perhaps exact birth date, etc. The transcriptions available online usually are only a summary of the main points.
I'm not able to help you search in Slovenia, but here's a little more from Minnesota.
Agnes's maiden name is given as KOZLAK here:
From Minnesota Birth Index at www.mnhs.org Proznick, Andrew BORN: 27 October 1903 Mother Maiden Name: Kozlak Certificate #1903-25041 County of Birth: St. Louis
From Minnesota Death Index Andrew Praznik DIED 27 May 1943 (not 1942) St. Louis County, Minnesota Minnesota Death Certificate #013926 Available for a small fee from Minnesota Historical Society: http://people.mnhs.org/dci/Results.cfmhttp://people.mnhs.org/dci/Results.cfm
From Minnesota Official Marriage System (MOMS). This index is NOT COMPLETE. An Andrew Proznik married a Barbara Hobly 27 June 1928 in St. Louis County A Lewis Proznik married a Frances Novakovich 07 February 1934 in St. Louis County
Researchers can track the children of Andrew and Agnes to find their death certificates, which could yield more about Andrew and Agnes - such as an affirmation of her maiden name.
There are a number of the Kozlaks listed on Minnesota Official Marriage System website (MOMS), but all in Hennepin County. Again, that index is not complete. No Koslars are listed.
Hope this helped a little! BTW, I'm a volunteer, not related