Humidity values returned by DHT22 sensors failing after some time
This is how it looks when they fail.

The DHTxx are very low cost sensors, optimised for cheap volume production. 

We don't know under what condition you actually use your sensors. Also, there's the phenomenon of knock-offs and low-grade selling.

In his comprehensive test, Robert J. Smith found a failure rate of 33% and concludes a mean life expectancy of one to two years.

A slightly more expensive alternative is the SHT series of temperature and humidity sensors. My personal experience is that they tend to tolerate some abuse (i.e. complete submersion in water) and don't age as quickly.

Link: 


failure analysis: http://www.kandrsmith.org/RJS/Misc/Hygrometers/dht22_first_failure.html
general writeup: http://www.kandrsmith.org/RJS/Misc/Hygrometers/calib_dht22_dht11_sht71.html#conclusions