perhaps it wasn't the med, just the natural fluctuation in symptoms? in other words, the PVC flare up or symptoms went away just as you went on the medication (by chance) so you attributed it to the medication (but it wasn't related). They are flared back up on their own and the medication never worked.
You need to increase the dose of the medication as your body will develop a tolerance, or adapt to, the presence of the medication so that you would need to take more over time for it to stay effective
In my humble opinion, take it for what it's worth, beta blockers are useless for benign PVCs in a structurally normal heart. It's the only tool doctors have, I don't even know how the tradition got started because it's off-label. There are 0, and I mean zero, studies to show that beta blockade can help reduce benign ventricular ectopy.
This also fits my own personal experience. I had a major flare back in 2009, 6,000 PVCs per day (casual rare PVCs my entire life up to that point). We tried several different types of BBs, and CCBs, and they all made it worse for me. I stopped the meds, my flare eventually resolved. I still flare from time to time. I don't understand it and it *****.