Just like whitetails, turkeys prefer a mix of wooded habitat with open fields. In short, they're creatures of the edge. And just like deer, turkeys will readily change their habits and locale when they sense even a small amount of hunting pressure.
Successfully hunting pressured turkeys in thick cover isn't necessarily difficult, but it does take a new approach to pull extra-weary birds within shooting range.
If you're having trouble getting turkeys to work your calling, odds are good they've been pressured by other hunters. Head to thick cover, calling sparingly and softly.