Oh dear. It does sound poorly. The lack of significant symptoms, like cough, sticky eye, limp, etc all I can think of is maybe anaemia. They can become anaemic if they are being bitten and feasted upon by mite.
For redmite, check the underneath of roosting perches after dark - wipe along the perch with a white tissue, it will come away with little red (blood) smudges if you have red mite. Northern mite actually prefers to live on the bird, check for mite/eggs at base of the feathers, particularly round the vent area.
