Play each note of the chord one after another (known as playing an arpeggio) to check if any are being accidently muted or need pressing down harder. If you find that you're accidently muting a string with one of your fingers, try lowering your thumb so the tip reaches around half way up the back of the guitar neck. This gives your fingers a better angle to approach the fretboard.
Guitars have been played since the Renaissance era, after descending from the ancient Greek instrument, the kithara. Or, at least, that's how the theory goes: the only real proof for this is the similarities between the Greek word and the Spanish word, quitarra. Early guitars often had four strings, evolving into the six-string version we know today in the late 1600s. Anton Stradivari, the famous violin-maker, also had a hand in making guitars. There's now only one Stradivarius guitar left in existence.