Snails

I still get them every day.

No, you should see them daily, but only IF your plants are fully watered (or close to it). If your plants aren't watered, you'll only see a watering option at the top instead.

If you use the "cup of water," as opposed to the "water" option listed in Actions, you won't see the snails. That might be the issue. I accidentally used the cup one time when I was in a rush, and discovered that afterwards, the snails never appeared even though it was time for them to. Make sure you're always watering from your can, and you should see the snails again.

I always do a raindance, and I get lots of snails.
OTOH, collecting them doesn't seem to have much of an effect on the plants. I did some testing, collecting snails from some plants and leaving them on others. They still grew up more or less at the same time.

Snails may appear later, but my experience is that you can either water your plant with the cup (when it is under 4 full drops) or pluck off a snail (when it has 4 or more drops) and once you have done one action, the other is unavailable until a certain amount of time has passed. Try just using your watering can to get a new plant to 4 drops or higher, and then see if the snail appears.

The option of removing snails appears roughly every 24 hours.

If you used the extra options to move the cloud or water the plants (both water and raindance count), snail will not show up in the next 24 hrs.

Picking the snail up reduces 2 hours of growing time each time you do it, it's very hard to have the exact timing so in some extend it is useless...but I will still use them every time...

I have occasionally had plants that never get snails, but it's pretty rare occurrence. And it has nothing to do with watering or time - all the other plants in the greenhouse would get snails, just this one particular one won't.

I just had the fastest snails ever - collected some christmas wreaths from the wild garden and had to remove the first snails after less than half an hour!

Oh, I always remove snails right away after getting seeds. The trick is to NOT use the cup of water that appears at the top. Water the seed normally with your watering can, then go back to the seed, and there will be a snail.

Yes, you get one "free gift" a day--either a cup of water to water with or a snail. If you use the cup of water, you don't get the snail. I just water all my plants with a game (when you win, choose the rainy clouds and every plant will get 1/2 drop of water), then I go picking snails. That way I don't punch the wrong "water" button and lose my snails.

Ah, that's interesting, thanks! You're right, I usually water new seeds with the free gift option, so that'll be why the snails only appear much later.
BTW, my most recent batch of plants has indeed shown some growth differences between plants with and without snails. Coincidence?