Did you do a parasite cleanse first? That's a must in the Hulda Clark Flush and well worth doing. I took herbal capsules for thirty days after my first (unsuccessful) flush and went on to have a series of very productive ones. No idea why that makes such a big difference, but it did for me.
Why lemon juice not grapefruit? Overall it sounds like you did follow the flush protocol and timing well, which is also very important. Don't start experimenting as that makes it less effective. Keep trying and make sure to always take malic acid and or apple juice to soften up the stones in the 7 days before the flush.