What causes low CD4 count despite taking ART for months?

Hello doctor,

My husband is HIV positive. I do not have any documentation or results with me. Our doctor says everything is fine, but we worry too much because of the slow CD4 recovery. All documents are in the hospital. The first test of CD4 was 280 cells per cubic millimeter. He was not able to start treatment at that moment. Two months later, he started treatment, and his CD4 was 75 cells per cubic millimeter. The first question is, what can cause this big drop in CD4? He feels fine without any other symptoms. After three months after starting ART, he was undetectable. After 9 months of ART, CD4 is still very low. The last result says 86 cells per cubic millimeter. He gained weight, and he does not feel anything is wrong, but anyway, we worry too much. Please tell us something and give us some advice and opinions.

Thank you.