It sounds like your Weather Station may need to be re-calibrated. To do this you need to remove the mechanism. While covering glass crystal with palm insert straightened paper clip or similar item into hole at center bottom of thermometer housing. While still covering glass crystal with palm, pry upward with tool to pop retaining ring out. Remove retaining ring, and turn thermometer over carefully removing glass. You may need to shake housing gently for glass to release. Pull up on housing, the hygrometer mechanism is the one toward the outside of the dial. You can recalibrate the hygrometer by wrapping it in a damp towel overnight and adjusting the pointer to 100% by gently twisting the mechanism with a small set of pliers. Periodically, the hygrometer will have to be recharged by this method of wrapping it in a damp towel.
To re-assemble, place thermometer housing in front of you (face side up), with the three ventilation holes toward you. Place new mechanism with tab resting in the depression at center bottom of thermometer housing. Place the glass crystal, dome up on dial face - be careful with glass edge, as it is sharp and remember to clean the inside of the glass, if needed. Place one end of the retaining ring over the hole at center bottom of thermometer housing, and carefully work the ring in under the housing lip all the way around until retaining ring pops in. This may require some pressure.
FYI: Some models have a foam gasket between the housing and the dial face. I was calibrating a TH-6 a few weeks ago and found that the gasket was pressed against the hygrometer coil, preventing it from moving. This could be the problem.
If you have any questions, please feel free to call us. We will be more than happy to walk you through it!