It doesn't matter. You can use Retayne on fabric that has already been washed.
There are a couple of different reasons why a fabric continues to bleed.
One reason is that the fabric has been over-saturated with dye; there is more dye than the fibers can absorb. After one or two washes, all of the excess dye has been rinsed out and you are good to go.
The other reason, which is the one you are probably facing, is that the manufacturing process did not properly and permanently set the dye. You can use Retayne to set the dye; just be sure to always wash the quilt in cool water afterwards, as Retayne loses its effect if the fabric is subsequently washed in hot water (even though you need very hot water to get the Retayne to work!).
Synthrapol merely suspends loose dye particles in water so they rinse away instead of settling into other fabrics. It does not permanently set dye, but rather is useful for getting rid of excess dye without having bleeds into other fabrics (or discoloring other portions of the fabric). This is what you would wash a finished quilt in if you are unsure about bleeding.