I'm not really sure how it's supposed to work, but when I use helm-projectile-recentf, I don't find the most recent files. Sometimes I do and sometimes I don't;)
I always find them with helm-recentf, though.
What are the criteria for files to be added to projectile-recentf?
Of course, the files are part of the same projectile, even the same directory, but if I visit a file, then switch to another file in the same directory, then try projectile-recentf again, the file is not there;)