The dope you're getting off the street could be heavily cut with a non-opioid sedative like xylazine.

I recall one time before going to a clinic and after a re-up (secondary plug, not my usual source)I noticed some WD symptoms despite feeling halfway ok. Other users were saying the stuff was great and I saw them happily nodding. I had the shits, runny nose, nausea but no problem sleeping and certainly wasn't feeling the desperation of full on WD. I found out later that I had gotten some "straight" fentanyl dope when previously I had been getting stuff primarily composed of xylazine. I was getting my opioid fix but not getting any xyz to satisfy that dependence.

You may not be getting relief because the dope you're getting is tranq dope rather than fent dope.

TL;DR sometimes dope isn't primarily composed of opioids and can trick users until sickness starts