A slightly stronger signal is needed for 'locking onto' a station, when tuning, than it is for day-to-day viewing. Your aerial is only of borderline quality when it comes to its ability to receive certain digital stations. Signal strengths at your aerial aren't constant; there was a strong enough signal for 'locking on ' when you first tuned the devices but not now. Add a cheap signal booster and you'll almost certainly fix the problem. (if not you'll have to upgrade your aerial system).
For a more detailed (and hopefully clearer) explanation, please see my post here:
http://www.theanswerb...y/Question789132.html
Chris