Okay, DeanoZ, I've got a little more time now so I'll take a shot at your hunt situations. Once again, my opinions only...should not be construed to be Gospel by any means.
First situation....you've heard a bird before daylight using a locator of some sort. You move in close....it starts getting light...but no more gobbles, or other turkey noises. Possibilities: you got too close and spooked the bird, or....you are dealing with an older gobbler that has learned not to gobble much on the roost. Either way, you are looking at a tough situation. If you don't think you have spooked the bird...i.e.--you didn't hear "putting" or wing beats indicating he has flown away, then you are likely on an old, wary bird, or one that has had a bad experience with a phantom hen recently. I have rarely seen birds that would only gobble one time on the roost unless they are suspicious of something. At any rate, calling to him on the roost, assuming he is still there, is likely to be counterproductive. The best course of action in this type of situation is to sit and be still and silent and hope that he gives himself away somehow. Patience is the key here.
If you hear other huntable birds in the distance, it might be best to head for one of them, especially if they are gobbling with any regularity. If you don't, then you should play the waiting game. If he is there, he will eventually come out of the tree...hopefully, you'll hear him glide or fly to the ground. Regardless, at some point....usually when its near full light, but before the sun is up....you will have to make the decision to call to him. Once again, with this type of gobbler, less is usually best....a couple of soft clucks is a good start, perhaps in combination with some discrete leaf-raking sounds (some of the experienced wing-users should chime in here).
If this bird is the only game in town, then patience and control over your desire to expand your calling is paramount. With this type of gobbler, uttering anything more than a few soft clucks or yelps, and maybe some purrs, at intervals every few minutes, is most likely a mistake. If he has not gobbled, or otherwise given himself away in an hour, then you should have by now assumed you are dealing with a very tough customer.
If he remains silent, and you have exhausted your patience for this duel, then you may make the decision to throw caution to the wind and begin some experimentation with your calling. Stay simple at first....a relatively soft series of say, five to seven yelps. See what response, if any, you get. In another minute, try another series, this time a little louder and with maybe a few sharp clucks thrown in at the start of the call. Next, try a short cutting sequence, followed by several louder yelps. If none of this brings a response, it's "Katey bar the door" from that point. He probably ain't comin'....and in fact, may well be on his way to parts unknown...so you may as well try about anything you want with him.
However, if you are hunting a small property, and this bird is your only real hope of success, you may want to forego the aggressive tactics and consider easing out and returning later to try to arouse his interest. Regardless, there are virtually limitless tactics and calling options that can be used at this point, and it is essentially a W.A.G. as to what is best to do, so just learn the different calls and stay after 'em. Also, the conservative tactics outlined here apply to this specific type of gobbler, and are not necessarily the best tactics to use with more vocal birds, two-year-olds, gobblers with hens, or any number of other circumstances and conditions.