GPS satellites positions change in time relative to earth (Galileo is different as it uses geostationary satellites). Depending on the time of the day, the satellite positions are different so if a location is ok for GPS one day then it might be more challenging the next day. For example close to tall buildings the satellites might be blocked by the buildings on one day, but another day more satellites might be visible because of their changed locations.

In open places the effect is not noticed, but within city streets it can be a factor day to day.