If the coloring in this picture is close to natural then I see a less than vigorous plant that is in need of chemistry (nutrients) necessary with which to do photosynthesis to make its own food/energy. Have you given this plant any balanced fertilizer at all? If so what and when?
The other issue is the amount of light. What is the light source for your cactus? What window? Is your cactus indoors all of the time? Is out of doors some of the time?
This might have been better as a 'comment', they'll let me know. Please give us a bit more info about this plant's normal environment, the plant zone you live within, fertilizer, type of soil, have you taken it out of doors for a sun bath? Need to know the amount of light this guy gets on a regular basis in its regular spot.