Probably the best suggestion that I offer my students when having chord trouble is to observe what your fretting hand is doing behind the neck of the guitar. Always make sure your thumb is placed behind the neck or slightly on the fretboard somewhere between your index and ring finger. If you find that your thumb is moving towards your headstock adjust it accordingly. I can't tell you the amount of times that students will get in the habit of pointing their thumb upward and that is a big handicap when trying to play chords. Unless you have some serious joint capabilities, I don't believe you will have many issues with pointing your thumb in the other direction.

Those two chords are always the roadblocks for beginning players. Might I ask how you are playing each of these chords? Are you playing the full F barre chord? Are you playing the B major chord with your route on the second fret of the A string?