From my experience, there is quite a number of effects that might influence the sensibility over time

• Catalyst 'wear': when using dopands, these may become inactivated by 'catalyst poisoning' or by diffusing away from the surface.
• These sensors are typically operated at elevated temperatures. Over time - expecially when heating more than 'normal' during a 'sensing sensation' - the crystalline structure may change.
• The sensor reaction with a agent may irreversibly alter the structure. 'Id expect this expecially to happen with reducing agents: too much of reduction will alter the sensor structure irreversibly.
• Excess humidity in combination with temperatures below the freezing point may 'crack' the structure.
• Similar things may apply if a dopant could be 'washed out' by humidity.
• Residue from oxidation or reduction may cover the surface permanently.
• ...