this is what i do in general, but i like when there are entirely different drawings for each adopt, and not just a color or design swap for each

id say change the line art
Also some of the designs are almost exactly the same but with minor changes and different colors, i'd say change up ur designs too

Me, personally, it's the price. Your prices aren't too high, I say, but I know someone who is cheaper. The thing about me which probably isn't exclusive, is that I seek price foremost, even before actual talent. I would rather pay $10 for a decent picture, as opposed to $50 for an incredible one. That's just me. I'm frugal. You're a very talented artist, but as aforementioned, I'm always looking for a bargain. Especially when it comes to adoptables. With them, you're not so much buying a character, but an actual design.

Personally I like when the designs are "unique", but I really like unique things.
Price depends, I'd be willing to pay more than usual if I really enjoyed the design. $5-15 is my range, usually, for adopts.
The thing I really dislike is cookie cutter adopts. I'm fine with the pose being copy/pasted, but hairstyle at least has to be different, for me. Preferably species and expression too. I'm also personally not much for commonly seen species like wolves, foxes, felines, etc. I like hybrids and unique species c:

Aah, also going to second ImWatchingU. I noticed some of the patterns being similar.
Also wouldn't hurt to have a few bases for adopts, I think c:

For me to get an adoptable I tend to get adopts where each character is unique instead of pallet swaps, you can use the same pose though.

You have to remember, people are buying the DESIGNS, so a person would tend not to come back after they got a design once from the artist.

though these days adoptables are oddly on the way out where artist now have to draw a single character with a unique design and auction them off instead of selling many of the same design with pallet swaps

In my case you have just happened to have saturated my market a bit. I bought 6-different vixen adoptables from you over the past year-- how many more do I really need? And while I do see some other adoptables of yours that I get tempted to purchase from time to time, without fail those will always be the only ones that are already taken. You can list 8 new ones and have only one of those sell, and absolutely 100% of the time, as if the other buyer had read my mind, the one that sold was the one that I would have wanted myself. *shrugs* But that's probably for the better though-- as I said above I have already bought 6 already, so I probably shouldn't be buying anymore anyway.