How to extract mouth numbing effect from Sichuan peppercorn into a syrup?
The reason you didn't get the numbing effect is because the hydroxy alpha sanshool is an alcohol that is largely soluble in oils not water.
When boiled it will come off in the steam and escape the boiling process in that manner. If you distill it, it will collect as an oil not as an ethanol/water soluble product. I doubt that putting the lid on the pot will collect significant amounts, or if they do collect enough to form droplets, they will be as fine oil layer on the surface of the liquid or on the lid of the pot.
I note that the extraction protocol mentioned on its wikipedia page includes ethanol and notes that the yield is low (~60%) of the available chemical. Boiling alone in water is not sufficient to extract any significant quantity.