Seems to happen to me quite often on the old vehicles I don't drive on a regular basis. Right now both my 72 F100 and my 68 Cougar exhibit that symptom on a fairly regular basis. The F100 will always start engaging again after I keep turning the ignition key. When my Cougar does it I have to crawl under the edge and rap the starter a couple times with a hammer and "loosen it up" and then it will engage again at least for the next few turns of the key. I plan on replacing both starters but haven't gotten around to it yet.