advice for breeding failure

We are looking for some advice form those with more knowledge and experience than ourselves.
About a year and a half ago we purchased 3 breeders, a boar and 2 sows. We let the happy threesome live on pasture together, no seperation. When the sows reached about 9 months old each gave birth to their first liter. One had 12 piglet and the other 6. Everyone grew up healthy and happy, we sold all but one ofhte first litter. That one then was bred back to the original boar (inadvertently) and had a litter of 3 at about 8 months old. Today all 3 original breeders and the one young mom are doing fine.. except...
the 2 older sows never got pregnant again. It has now been a year since their first litter, and no piglets. I have seen each of them in heat multiple times over hte last year, and noticed the boar mounting one of them at one point. Otherwise he doesnt seem interested in them. He of course did mate with the smaller younger sow as soon as she went into heat. The boar will sniff around the sows from time to time but we dont notice him trying to do anything more.
There are no signs of disease or stress, so we are perplexed about how to get them breeding. Is it necessary to separate the boar and put the sow in with him for breeding? This doesnt seem like natural behavior.. as they wouldn't do this in the wild, right? Why would e breed with the young sow at first heat but not he older sows?
Any advice is appreciated. We truly want to raise these guys in as natural an environment as possible with behaviors as in the wild. All else is working except the breeding.
About a year and a half ago we purchased 3 breeders, a boar and 2 sows. We let the happy threesome live on pasture together, no seperation. When the sows reached about 9 months old each gave birth to their first liter. One had 12 piglet and the other 6. Everyone grew up healthy and happy, we sold all but one ofhte first litter. That one then was bred back to the original boar (inadvertently) and had a litter of 3 at about 8 months old. Today all 3 original breeders and the one young mom are doing fine.. except...
the 2 older sows never got pregnant again. It has now been a year since their first litter, and no piglets. I have seen each of them in heat multiple times over hte last year, and noticed the boar mounting one of them at one point. Otherwise he doesnt seem interested in them. He of course did mate with the smaller younger sow as soon as she went into heat. The boar will sniff around the sows from time to time but we dont notice him trying to do anything more.
There are no signs of disease or stress, so we are perplexed about how to get them breeding. Is it necessary to separate the boar and put the sow in with him for breeding? This doesnt seem like natural behavior.. as they wouldn't do this in the wild, right? Why would e breed with the young sow at first heat but not he older sows?
Any advice is appreciated. We truly want to raise these guys in as natural an environment as possible with behaviors as in the wild. All else is working except the breeding.