GPS (Global Positioning System) can stop working or experience reduced accuracy in certain areas for a variety of reasons:

Signal Obstruction:
Urban Canyons: Tall buildings can block or reflect GPS signals, causing inaccuracies or loss of signal.
Dense Foliage: Thick forests or vegetation can obstruct the line of sight to GPS satellites.
Mountains and Valleys: Natural terrain can interfere with signal reception.

Interference:
Radio Frequency Interference (RFI): Signals from other electronic devices or communication systems can interfere with GPS signals.
Jamming: Deliberate jamming devices can block GPS signals over a certain area.
Solar Activity: Solar flares and other space weather events can disrupt GPS signals.

Multipath Effects:
Signals can bounce off surfaces like buildings, water, or the ground, causing delayed signal reception and inaccuracies.

Satellite Issues:
Satellite Maintenance: Periodic maintenance or outages of GPS satellites can affect signal availability.
Orbital Issues: Problems with the orbit of GPS satellites can affect their coverage and signal strength.

Receiver Problems:
Hardware Malfunction: Issues with the GPS receiver itself can lead to signal loss or inaccuracies.
Software Issues: Bugs or outdated software in the GPS receiver can cause problems.

Environmental Factors:
Weather Conditions: Severe weather, such as heavy rain or snow, can attenuate GPS signals.
Atmospheric Conditions: Variations in the ionosphere and troposphere can affect signal propagation.

Geographic Location:
Polar Regions: GPS coverage is less reliable in extreme northern and southern latitudes.
Underground or Indoor Environments: GPS signals generally cannot penetrate solid structures, so reception is poor or nonexistent underground or inside buildings.