Good morning Warmfish.
If this is correct ?
"Several times I broke off just below the knot, with the loop still intact."

Then there is not a problem with the knots, there is a problem with your leader material, the Fluoc.

A two turn surgeons.
Myself, I would never even dream of a two turn knot with FC, I always use a four turn.

Two turn is ok in monofilament leaders, but not FC.
Because FC will not compress, rather it will cut its self off !

The other knot for FC is the Slim Beauty, which is the best of all for FC.

Kind regards,
UB