The market changes all the time, especially after a new DLC or new expansion. Like right now, most players are spending their gold on Blackwood related things, like Companion gear or new Blackwood recipes, so they aren't buying a lot else, except maybe mats to upgrade gear with the latest meta. You just have to find something else that sells. Also, there are potions that still sell well, like Essence of Spell Power, but a lot of other people are selling those too, so you have price competitively. You can always just sell the alchemy mats. But being dependent on one thing will eventually mean the market changes against you one or way or another--either people stop shopping for that item or so many people are selling it the prices drop. Adapt or die.

As for Crown pots and poisons, the only crafted potion those really undercut is tri-stat Essence of Health, and that never sold that well anyhow. But Essence of Spell Power, Spell Crit, Weapon Power and Weapon Crit as well as the magicka Immov pots still sell, at least on the NA PC server, although a lot of players know it's cheaper to get the mats and craft them themselves so some weeks potions just move slow.

As for poisons, that's always been a fickle market, which is why I have never bothered with trying to sell them.