Silverbrooks
I give jars a fairly sharp bang on the countertop, lids sideways, to release the seal on shop-bought jars. Sometimes you can hear the seal release. Wear something to protect your eyes and maybe rubber gloves just in case the jar shatters although it's never happened to me. Have you tried that already?
Since reading of such methods here on Gransnet, I always resort to them now when necessary,
Never again have I needed to rush out and accost my neighbour on our shared front path (actually, ex-neighbour, though can't think why!) as he arrived home, crying "thank goodness you are here - I need a man"!