Well, three times is not a huge sample. When I last ran DR lairs for SES, I was seeing drop rate of around 25%. And as usual the RNG can be streaky. Sometimes I get 1/8 runs, sometimes 6/8 runs.
The reset time changed/broke a long time ago. The quest reset happens around 12 noon US EDT now. On the weekends, make sure you get the, "pick a lair" quest to be sure you got the quest eligible for SES.
The Dread Ring weekend schedule (Liberation) which grants you stones from all lairs do not start in sync with the daily reset. I am pretty sure when you did the runs, you still had Friday's schedule (Necromancy) up, which only grants scrips. Always check which schedule is active when you're in the zone.
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