Not enough satellites
GPS devices typically need to receive signals from at least 7 or 8 satellites to calculate location to within about 10 meters. With fewer satellites contributing, the amount of uncertainty and inaccuracy increases.
With fewer than 4 satellites, many GPS receivers are unable to produce any location estimates, and will report "GPS signal lost". The GPS unit is usually looking to find and acquire signals from more satellites, so should eventually recover from situations where not enough satellites are found.
Initialization and warm-up
GPS receivers need some time to first acquire signals from satellites. When investigating GPS accuracy, try to allow plenty of time (5 or 10 minutes) for the receiver to acquire signals from satellites.
When first turned on, the GPS needs to download data from the satellites that describes the position and timing of all of the satellites in the system. This helps it to acquire signals more quickly in the future. This initial cold start can take 5 or more minutes.
If you use a GPS device frequently, the device will have up-to-date information about satellite positions and timing, so should acquire signals from satellites within about 1-3 minutes.
Assisted GPS (AGPS) start
Mobile phones take advantage of extra location information to bootstrap GPS initialization. This is called "Assisted GPS" or AGPS, and uses information from the mobile phone network's cell towers to provide a rough starting point.
Typically, when using AGPS, a receiver can acquire satellite signals in 10-30 seconds. After the initial acquisition, the GPS receiver uses only the satellite signals and no longer needs assistance from cell tower information to update its location estimates.
Buildings, trees, tunnels, mountains, clothing, and the human body can prevent GPS signals from the satellites reaching the receiver. When possible, put a GPS receiver in a place where it has a clear and unobstructed view of a large portion of the sky.
In some cases, this can be done by holding the GPS device in a back pocket, or on the outside pocket of a backpack, or in a handlebar mount. In other cases, the obstructions are unavoidable, like in downtown areas where tall buildings block view of the sky, or when in dense trees.
When signals from the GPS satellites bounce off buildings, the GPS receiver can be confused by the extra time the signal took to reach it. In these cases, you may observe sudden large errors in position. There is not much that can done to reduce the effects of multipath errors - GPS is simply less accurate in canyons, whether they are made of buildings or natural formations.