I did two celestial dungeons on the first day they came out and got one sigil each. I have not gotten a single sigil since, despite doing at least two celestials every day. Is this intended behaviour? Am I just uniquely unlucky?
And yes, I do manually loot the bosses.
I have done a LOT of them(like, really). I have tank gear and heal gear all bought with these stone thingies, complete. I have bought multiple bags for the mounts(only one so far).
And i am still missing 2 sigils. Sometimes they drop pretty ok, most of the times not.
Thats just how it works 
On my mage main I have 9 sigils of power and 0 of wisdom wich mostly came from H MSV than celestial dungeons, but my guardian druid alt, whom I prepare for challenge mode dungeons, is already at 5/3 sigils.
Over the span of like 20 dungeons, I’ve gotten 1 sigil in total.
So… I don’t think you’re super unlucky. They’re not a 100% drop.
got 5/5 on my main and 3/4 on my twink, both not valor capped…
Yeah more like 25-33%
You have like 3 more weeks to get them since you wont be able to complete the next part of the quest until Terrace is open anyways, so it’s not a huge rush.
I have 2 sigils of power and I have only done raids.
I am done total of 140 dungeons in Pandaria (all dungeons included) of those i can say that roughly 110 is Celestial dungeons and i am at 10 power sigil and 7 Wisdom. Celestial dung has been out 4 days now and i am already starting to hate them. Human nature (or atleast mine) is try get quest done soon as possible, even i know that i will need to wait next phase. I would totally lose my mind if i had to farm sigils from celestial dungeons more than 1 char. Blizzard could/should have added small drop % for daily celestial dungeon quest/ Satchel of celestial chance