Soooo frustrating! Why are they not visiting?
When you replant, you might want to consider more than one herb. The lops are rather rare, and the brown bunny will beat them off almost every time.
If you are going to use just the one herb, consider using a fully grown blue or yellow Bellis to deter the bunny, and a RTH Rosa Amora to deter the foxes (which the bellis attracts). Combined with a mushroom to deter some of the mice and ducks (preferably a supershroom) that's not a bad setup. Otherwise use more herbs to increase the attraction for the lops, whilst accepting that you will get brown bunnies alongside.
Actually they are only attracted when the Thornbloom is RFH:
"How To Attract
Only a ready for harvest Thornbloom Herb will attract the Lop Eared Bunny."
Tracey's second post has good advice - I think the problem is just the amount of competition for a rare critter.
Sorry, I was thinking of the hare. It's too early for me! I'll remove that post because it's confusing otherwise.