I have talked to several doctors about this and the truth is they don't know why meds don't work as well. I have twice left the hospital on antibiotics only to end up back in the hospital with a horrible infection, needing IV antibiotics.
Part of the problem is the area that is bypassed is where we absorb most of our medication. The same thing happens with alcohol, it goes into our system fast but is gone just as fast!
I noticed the pain meds after my lower body lift, I was in HORRIFIC pain, and had to take 2 percocet every 3 hours (as prescribed by my doctor), but just a couple of years before my WLS I had had 3 c-sections and recovering from them I had 1 percocet every 6-8 hours, and that did the trick.
You may need to find a different medication, or a medication that is given differently, like in an injection, inhaled or in a patch. For my migraines I have the up the nose, injections and one the melts under your tongue and they all work better than the ones I take orally.
After my lower body lift is when I started asking questions about absorption and medication. I have been told that they are currently doing studies on this, if that is helpful at all.
Hopefully you can get some pain relief, soon.