RNG, Luck bla bla bla is all it comes down to when farming for Signet lolz. Personally i had farmed for 1 whole week when i was PL50 but has yet to find 1. Gave tips on how to farm signet to few players and they found it in a matter of days.. haha.
The invi snake is the best bet as their mlvl match signet ilvl. Here is what i usually do to farm signet (if its any help to those still trying)...
-Save checkpoint in VotA to get 5 nv
-After getting 5 nv wp to path and go into sewer .. clear sewer.
-Oasis WP and run around oasis (up to u to kill every mob along the way or not but focus on those invi snakes) look for places that spawns a lot of those invi snake (cellars, the 2 random tombs) and finish by clearing Forgotten Ruins.
-Repeat n Rinse.

Yup VotA is Vault of the Assasin .. easiest/fastest place to get 5NV
20 runs? i hv farmed signet way more than that and guess what ... still no signet haha
Keep on grinding & pray RNG Gods are smiling on u