This happens to me too, but I'm sure it's because i abuse everything to death and take obscenely high doses. I overdosed off of both downers (heroin that needed 3 doses of narcan) and uppers (IV cocaine that caused me to stroke out and go into a seizure) when i was younger, so I'm always chasing that extreme high. If I'm not so high that I'm in medical danger i feel like it's not working because i remember those feelings and know how high it's really possible to get.
I stopped using street drugs ten years ago out of fear I wouldn't make it past 23. Now i heavily abuse prescription pills including opiates and amps, and have convinced myself that it's safer because I'm not using a needle anymore, and i have a doctor giving them to me (well the amps aren't mine but that's an occasional thing) so it's ok and i need them, but i have gotten extremely reckless with those to. I take huge doses and mix tons of things together in an effort to mimic the overdose highs (hopefully without the almost dying part).
I was clean off of everything for 6 years until a back injury required prescription opiates. I wish i never started back down this road and am afraid eventually it will lead back to the needle.