Do you have hard water? My henna used to dull over time until I used a chelating shampoo to get rid of the mineral deposits. The henna was still the same colour underneath.

If it has got lighter is it possible the sun has bleached your hair? Mine only lightens through sun and environmental damage (so I cover it to protect it from the elements).

My hair is medium chestnut brown, for reference. I haven’t hennaed for nearly 4 years and the colour has stayed the same.