Are your employees competent? Are they qualified to make these decisions? If they are, then the reason that they are choosing to avoid responsibility probably stems from them having received a message from you somewhere along the line that you want to be in control. This was probably not a verbal message. Somehow your actions have shown them that you don't really want them making the decisions. If you have given them a conflicting message ("take charge!!" vs "you can't do anything right, let me do it"), they will follow the unspoken rules, not the public policy. Has there been a time when someone did make decisions and you let it be known that you were not happy with his or her choices?
When you are not at the job site is it clear who has authority? Is there a clear chain of command? This is key. Someone must be appointed supervisor. Appoint someone and then mentor him or her so that she or he understands what you want and can provide it to you. A quality supervisor is one who will be willing to take charge while also knowing when to ask for your help.
There are two sides to every story. I have a feeling that you would be
shocked to hear your employees perspective on this situation. If all else fails consider bringing in a mediator who can facilitate a dialogue that will allow you to get the broader view.