My first thought, how long was the bottle opened from your first dram from the bottle until you had it and didn't like it as much?
Very few in the industry know enough to submit to the fact that spirits do go bad once opened. However, THEY DO GO BAD!
Depending on filtration, cask marriage, storage conditions (and this means from bottling, transport to market, time in distribution warehouses and retail shelves to the drinker's bar/home environment, hot/cold/light exposure) spirits can change negatively in bottle. Given a good bottle at opening, once a bottle is opened and has air contact, unless it's being preserved, you have 3-6 months to drink it at its best. Sometimes 2-6 weeks for more unique single cask-type bottlings. There are certain aldehydes that develop with air exposure that can, and do, negatively affect the spirit.
The problem is most people in the industry don't know this, and can't recognize it if they do! In fact, after 12 years in the industry and 30 years studying products, I've only met a handful of people who know this and purport it in their business dealings.
Phillip Hills' book Appreciating Whisky goes into this in some detail. He is the only one I've read who's drilled this point heavily!
The fact is, big peated malts like Lagavulin have so much going on in them that there is a lot that can go wrong, but people probably notice any issues in them less because of all of the things going on compared to say a bottle of Dalwhinnie or Glenkinchie that's been sitting open on a bar's shelf for three years.
My point being, it was likely not you but was likely your whisky.
My advice, only keep open what you can drink in 2-3 months. As opposed to having 100 whiskies and all of them open. For those like me whose bottles are part of their drinking collection, keep the empty bottles. I get through 6-10 bottles of whisky a month so I routinely will have 8-12 bottles open at home. I've toyed with buying a preservation system but now simply mark the date I open a whisky on the bottle's label and drink up in a timely manner. I call it self-preservation.
Living in New York, I have frequented most of bars with better whisky selections. None of them have anything in place for product preservation. I've subsequently had terribly drams poured me at Brandy Library, Keens Steakhouse, St. Andrews, Caledonia to name a few. I've also had the same issue at whisky bars and pubs in Scotland. The only two that have ever done anything when pointing out a bad dram is the Highlander Inn in Craigellachie and the Mashtun in Aberlour. So rather than try a rare whisky that's been open for 10 years in a pub, I tend to drink what's been opened most recently when I'm out.