It took me forever to find Ulfert Behrens in the 1860 and 1870 census. The problem was partially solved when I learned how he likely pronounced his name. This low-German native probably said his name something like "barns." Here I was thinking it would have been pronounced "Bear unds" (rhyming with roughly with "errands"). Once I started looking for names that sounded like "barns" I found references I had previously overlooked.
Find out how your ancestor likely said his name--you may get variant spellings that you never thought to look for.