In my experience, poppers are always most potent when the bottle is first opened. Depending on how long you keep the bottle and how many times you've opened it (over the course of days/weeks), the potency degrades noticeably, although perhaps not completely. Usually by the time you get to the final 1/4 of the bottle, it's probably time for a new one. Poppers, in essence, do go stale.
The other thing I've noticed is that (at least in the U.S.) the formulations have changed repeatedly. From what I've read, poppers are in a gray zone, legally, and have to be reformulated each time the then current formula is made illegal.
It's also possible, if you're using them often, that you're developing enough of a tolerance that the initial rush you've experienced is not as pronounced as it was when you first tried them.
My sense is that the poppers available today are not quite as amazing as those that I first experienced a decade or so ago. But, I still enjoy them, especially when they're fresh.
It's a shame, though, that our enjoyment of poppers (and so many other things) is held hostage by know-nothings that make it their business to ban them, but that's the all-too-hypocritical nature of the world we live in.