I have Chronic Pain Syndrome over my whole body and used oral hydromorphone for years at very high doses w no side effects.
I had a pain pump installed targeting my lower back down to my feet after a successful trial.
My provider was new and after a year was still at a very low dose (less than a microgram per day) and I had little to very little relief of pain. I switched to another provider who increased my dose aggressively (up to 7.3 micrograms per day over the next year) and I received enough relief in my feet and legs to be able to walk/stand most of the day.
It’s been 3 years now w no relief to my lower back. After a catheter test to ensure proper placement & some genetic testing, they switched me to Fentanyl. We’re still titrating up but have had much less relief that on the hydromorphone and no relief still in the lower back.

My point is not that it won’t work on backs, but that we all react differently to meds and providers are scared to increase the dose even when it is blatantly obvious that that is what is needed.
If you are not having any side effects and they are holding your dosage down, ask them about it and if they do not want to increase it to see if it can be more effective then find another provider - but interview them first.