Hypocellularity in Bone Marrow

I am wondering if anyone could give some input on what could be causing this for 10 year old child.
Child has had reticulocytopenia , mildly low hemoglobin (around an 11) and low borderline low RBC count. WBC and platelets have been normal. These CBC results have been ongoing for 18 months now. They have stayed at a consistent baseline. Many tests have been done to find the cause of this including all iron and nutritional testing, genetic testing, viral testing, inflammation, etc. A recent bone marrow biopsy showed a result of
Hypocellular bone marrow showing trilineage hematopiesis. 40% cellularity. No cancer or malignancies.
We are all puzzled as to what could be causing this. Child is growing normally and energy levels are OK but has started frequent migraine headaches within the past year and “seeing stars” recently when running and jumping on trampoline.
Any advice appreciated!