I'll bet it wasn't cooked long enough. If you have a Vitamix, you can add
several yolks and the thin curd and whirl a few minutes (~4 min) until steam is rising.
Conversely you could warm the curd on the stove, temper the yolks, add them back in to the thin stuff and cook until 170 degrees, stirring constantly.
My favorite recipe uses 5 egg yolks for 1.5 cups of curd and other uses 5 whole eggs for 3 cups of curd.
I think I'd try adding 4-6 additional yolks. Or...you could just try cooking to 170 degrees and see if it thickens on its own.
(Qualifying statement: I've never done this. Just speculation)