Your sow might be too old and you may have left your gilt too long - but if they're in with the boar you've got the best chance of success
I think it's a case of
sometimes these things happen. It's not THAT long since your sow had piglets so there's a good chance of her getting in-pig, same with the gilt - if she had been 2-3 and never had a litter I'd be sceptical, but at 16 months she should be fine. As long as they are healthy and not over fat, you're giving them the best possible chance. Also you probably won't see the boar serving the pigs - knowing when they are in season and watching if they come back into season 3 weeks later will tell you if he's been successful - but it may take one or two cycles before this happens.
Having only recently started breeding (pigs, anyway
) I'm not hugely experienced, but it's my understanding that (in commercial production, at least) they keep the sows breeding regularly to increase litter size - the number of piglets drops with the length of time between litters- according to my friend who works in a commercial piggery.
HTH
Karen x