Here's an example of something weird. I know that Thomas Gell married Anne Kennaugh 11 Apr 1857 Malew. I went into the new Family Search to see if the fathers are listed (one of the nice additions to the new site). My search criteria: "Thomas Gell", year "1857", marriage place "Isle of Man". Not there!
Went back to the old site and it's there. Went to new site and took out marriage place "Isle of Man" and left it blank. Huge list appeared which included the one I wanted. Substituted marriage place "England" and it's there. So, by including the place "Isle of Man" it actually rejected it because it wanted "England". This isn't universal but it might be a reason you can't find something that ought to be there.