How many drops are you adding at once? I had to cut down to 2 in a 510 and 901, and 3 in an 801 to avoid leaking. I Immediately vape after dripping and don't drip again until I taste a change in flavor. I'm also losing the habit of standing my mods on end, especially if the drip is still kind of fresh, I put them on their sides. Already have a habit of blowing the atty back out and wiping threads every so often.
I basically had to adapt ways of avoiding a leak. I am personally liking the 510, 401, 901 and 801 for dripping, each for it's own way of burning and tasting. They all give me the same amount of leakage if I use too many drops too quickly, or leave them sitting too long with a fresh drip.
Do you clean the 901 atty out? Maybe the wick's caked up, keeping the juice from absorbing?