RNG is RNG... For my oakensoul ring i got it within like 2 attempts of all the required things for instance.
based on what i've seen from other complaints...
do leads when you get them, don't sit on them
if you allow a lead to expire, it may be possible that the lead is locked from you getting it again- if you feel this is the case contact support (ex- in cases where you have killed something thousands of times without a lead drop).
there haven't been many reports of people saying that because they have all other leads for an item, the last one is harder to get. If you want to count this as more evidence against this- I did Banished Cells 2 like 10 times with 3 of the leads for Syrabane's Ward, decided to take a break and go fishing in Galen for one of the other leads, and got it after 3 attempts. That doesn't seem like a reduced drop rate to me. I would like to think that drop rates are set at a low amount, but anything is possible.
when I had to kill that beholder mob for a different lead, I probably killed more than a thousand on my sorc, then switched to my NB and got it within 10 beholder kills. It is probably superstition to suggest that switching up characters leads to better lead drops, but changing up your characters can be a healthy changeup in otherwise monotonous farming.