My mum has a gilt and boar. The boar turned 2 in November and the gilt turns 2 in April. They have been together, they have been apart, and together and apart about 3 or 4 times, my parents have now given up hope and they are together permanently if they so wish (they have a hole in . No piglets yet. We've had him from 10 weeks old, we've had her from 5 months old, put together when she was 8 months old, she cycling every month, she flirts with him, she even gets really angry when he doesnt do his deed and storms off/gets grumpy with every one. He shows no interest, he will smell her, and flirt through the fence but you put her in with him and he does nothing except be grumpy as she brings all the dirt into his house and doesnt take it back out again.
There is the other answer to send one of your gilts out to a different boar, (you can send both but if they both get pregnant then you have a lot of piglets to deal with) and hope once she has some experience she can lend a helping hand to the boar. (my dad doesn't want to do this with our gilt). Other than these two pigs, all other larger pigs I've seen, seem eager to breed.
So doesn't answer your question, but we do have the same problem.