Dry is not always wrong
Some species thrive with a mostly dry surface and a full water dish. The question is whether the enclosure is too dry for the particular spider, not whether the substrate looks dry in a photograph.
Higher-risk situations
Tiny slings, moisture-dependent fossorials and warm well-ventilated setups can dry quickly. A repeatedly empty water dish, rapidly shrinking damp zone or visibly dehydrated spider deserves attention.
Add water to the enclosure, not the spider
Restore moisture by wetting part of the substrate or overflowing the water dish where appropriate. Spraying the spider itself does not fix a dry substrate profile and can cause unnecessary disturbance.
Use a gradient
A moisture gradient gives the spider choice. Damp lower layers with a drier surface are often more useful than trying to keep every centimetre equally wet.
Ventilation balance
If the enclosure becomes bone dry within hours, airflow may be excessive for its size or the substrate may be too shallow. Fix the design rather than watering constantly.
Watch behaviour in context
A spider hiding or sealing a burrow can be normal. Use body condition, access to water and the known needs of the species rather than one behaviour as proof of dehydration.