Some causes of GPS signal weakness or loss
A GPS unit is only going to work if it gets signals from several satellites - and these must come by direct line of sight.
Mountains and buildings can block the signal, although mountains are often not steep enough to obscure a satellite.
Valleys are often bad, epecially if their sides are covered in leafy trees.
Trees in leaf are a real problem - sad because we would all like to have some navigation in thick woodland! The water in the leaves absorbs some of the satellite signals before they reach the GPS. The newer Garmin units are much better than earlier models at working with weak or intermittent signals.
Thick clouds can increase problems. Clouds consist of water droplets that will have some signal-absorbtion ability. There is very little reference to this in the literature. My own experience, especially with older GPS units, is that there can be increased problems on gloomy days under deep thick cloud cover. Also I have heard of GPS-controlled drones getting lost under thick clouds.