There are several factors that may affect your GPS quality. For instance, riding into woods, in urban canyoning or in cloudy weather may disturb the GPS signal communication between your smartphone and the satellites.
When your GPS tracking starts providing inaccurate location or other data while tracking your activities, this state is named "bad GPS." This may result in inaccurate or missing activity tracking data.
If your device loses its connection to GPS satellites, this state is called "no GPS." In this case, no location data will be recorded.