You're likely just relatively GH insensitive. This is not cause for alarm, you do deviate from the mean of the normal distribution of IGF-1 serum levels, but that's OK.
Circulating GH-binding protein (cGHBP) levels, I would think yours are fairly low, are generally reflective of GH receptor sensitivity as GHBP is derived from GH receptors, basically they are cleaved off from the GH receptor at the cell membrane [1].
"At low GH concentrations, when GH receptors are in excess of free GH, a comparable response to a given amount of GH is expected in all subjects, whereas at a higher GH concentration, the response becomes correlated to the expression of the GH receptors (by levels of cGHBP)" [1].
As we know, at your normal levels of GH and IGF-1, you're healthy. That's what matters, not some arbitrary range. But GHBP levels are the greatest contributor to the wide variations in individual response to GH.
You do not need an endo to get your IGF-1 levels up. Hell, IGF-1 is not something you want chronically elevated unless you are banging hGH and YOLOing like some.
Basically, this just comports with wide interindividual variation, and your observation that you may just be a "poor converter."
[1] Hansen, T. K., Gravholt, C. H., Ørskov, H., Rasmussen, M. H., Christiansen, J. S., & Jørgensen, J. O. L. (2002). Dose Dependency of the Pharmacokinetics and Acute Lipolytic Actions of Growth Hormone. The Journal of Clinical Endocrinology & Metabolism, 87(10), 4691–4698. doi:10.1210/jc.2002-020563