Your oil glands have gone on strike!
The meibomian glands are located in the eyelids. Secretions from these glands comprise the lipid (oily) layer of the tear film which is so crucial in preventing rapid evaporation of the tears. Failure of these glands to produce or secrete oil - due to chronic blockage, thickening of the meibum, etc. will affect the quality and stability of the tear film, which in turn will produce classic dry eye symptoms even in people whose dry eye test results appear normal. A TBUT should show whether the oil layer of the tear film is adequate or not.
Meibomian gland dysfunction is quite common and unfortunately it often goes undiagnosed. Even when diagnosed is often not treated or is not treated effectively until it has become chronic or severe.
MGD may be inflammatory (often as a result of blepharitis) or atrophic.
For more non-technical background to put MGD in context, please read Dry Eye for Dummies.