Turkeys can be spooked by excessive poor calling. Gobblers have keen hearing and are especially attuned to subtle calls made by hens. If you're not confident in your calling abilities, concentrate on making periodic soft yelps, purrs and clucks. Try calling for 30 seconds, then wait 15 minutes. If you can see toms, gauge their reaction to your calling and respond accordingly.
Turkeys are roamers.
With a little patience, you often will be able to entice them within range. When you move, do so quietly and stealthily. Remember that an approaching tom may be able to see you without you seeing him. Birds approaching you from behind sometimes will walk right past you if you're steady. Turkey silhouette targets are available at most sporting goods stores.