When it comes to manes, there are 3 possibilities, no mane, single mane (SM), and double mane (DM). A rabbit with no mane has two copies of the maneless gene. A rabbit with a double mane has two copies of the mane gene. A rabbit with a single mane has one copy of the mane gene, and one copy of the maneless gene.
When a maneless rabbit is bred, it gives a copy of the maneless gene to all of its offspring, because that's the only kind of gene it has. Similarly, when you breed a double-maned rabbit, it will give a copy of the maned gene to all of its babies, because that is the only kind of gene it has. But when you breed a single-maned rabbit, it has 2 kinds of genes (one for mane, one for maneless) and it will give the mane gene to some of its babies, and the maneless gene to others.
When you bred your single-maned Lionhead to a maneless rabbit, the maneless rabbit had to give the babies a maneless gene. The Lionhead gave a copy of the mane gene to some babies, and the maneless gene to others. Babies that were born without manes have two copies of the maneless gene (one that came from the mother, and one from the father), and no copies of the gene for manes; they never will have manes, and the only way they could ever have babies that have manes would be if they were bred to a rabbit with a mane. The babies that were born with manes are single maned. SM Lionheads often have fairly good manes as babies, but the mane tends to thin out as they molt and grow in new coats. By the time those babies are adults, they will have considerably less mane than they did as babies; perhaps as little as a few tufts of longer hair around the ears and on the jaw line.