I'd say about a week ago I could find alchemy orbs no problem. Recently, I've been on a very dry spell. It's been about a week without finding one. I'm going at the same pace and I'm doing the same areas. I find sparks of life at a decent rate but still no orbs of alchemy. It seems as if the drop rate changed, but I looked in on the price of the orbs and they are about the same. I'd assume the price would raise if they were more rare or something.
I'm just really confused and bored lately. I thought maybe it was some strange bug and I've tried different areas and I even switched up my equipment thinking that had something to do with it somehow.
I found 3 Orbs in a single run today while heating up my random gear. Whenever I purposely search for orbs, I always find that I don't find any. Coincidence? Maybe.
Sounds like you're just having bad luck. And if it takes you that long to find them, there's always the Supply Depot. It's all a matter of what luck wants to happen first, you being able to find the orbs, or you being able to afford to purchase the orbs.