You don't say where exactly you are and what your climate is but a Meyer Lemon would be much happier outdoors for as much of the year as possible. Other than that I can only suggest you take some pictures and show us what the problem looks like. There may be a watering problem, a nutritional problem or a pest problem. Lemons are martyrs to scale indoors. Or alternatively post over on the Citrus Forum.

I had a Meyer for over 15 years in a large pot. It spent March to October outdoors. It was watered and fed with a citrus feed and repotted about three times in that period. It always perked up no end when it was put outside in the Spring.