It could be a number of factors, but improper storage is the most likely reason IMO. Even if you are only smoking a few cigars a month, the cost of a hyrometer is minimal.
Western Caliber III:
http://www.bargainhumidors.com/bh/Pr...d=ACC-CALIBER3
$19.99 plus shipping
Little Havana:
http://www.littlehavanacigars.com/pr...?prod=HY1062LT
$17.95 with free shipping!
I have a number of Western Caliber II hygrometers (the older model) and have had a few issues with them. I have heard excellent things about the new Little Havana hygrometer and would probably give that a shot next.