I've got some Szechuan/Sichuan peppercorns that don't make my lips numb or tingly, even when I just chew on several of them for 10 seconds or so. I would expect that they would. The jar smells good. Are they old? Low quality? (Or am I immune to the chemical?)
I wouldn't say that Sichuan peppercorns (zanthoxyli pericarpium)，hua jiao (花椒）go stale, but they do lose some of their numbing effect/potency the drier they are. Fresh ones are green in color, quite difficult to find in USA, and tend to be the most potent in their numbing power. There are reddish ones which are what we normally see here in the USA, which are pretty potent. There are light brown ones, which are a bit more dry, with less potency.
I would say the drier they are, the less potent they are in their numbing power. But the drier they are, the easier it is to powder/crush them if you just want a bit of the numbness.
You could also lightly crush and heat them in dry pan or you can add them to your dish right away if they are dried out. Of course, add more to increase their potency.
I would suggest a dark/opaque, air tight container in the fridge to help preserve their potency.
It's impossible to say for sure as senses are subjective, however there may be nothing wrong with it. Sichuan pepper comes from a completely different plant than black pepper, it's actually in the citrus family, and is more aromatic than spicy. You should get more of a spicy, slightly citrus note rather than it blowing your mouth out with pepperiness. If you aren't tasting much they're most likely old, spices do lose potency over time. Keeping spices sealed well in a cool, dry place helps slow degradation, but eventually the flavor will go.
I believe all spices and dried herbs or seasonings lose potency if stored improperly, I would recommend trying to store them in an airtight container. Also toasting your spices on the stove top in a dry pan over medium low heat until they are frangrant will also help increase the flavor you get from them. Hope this helps.