Why don't you come up and see me some time ?
This can be a very vexing problem and I, and friends of mine have ended up with more guinea pigs than they intended to have because we got it wrong!.
In the main, guinea pigs are a pretty fertile bunch and more often than by the simple procedure of putting a boar with a sow you will more often than not end up with some offspring in the none too distant future!.
However, sometimes you will get a pair who simply do not prove fruitful. You won't know if the problem is with the boar, the sow or both, so the only way you will find out is to change partners. However, if, say the sow goes onto have a litter by another boar don't presume that boar number one was a dud, so to speak and the same goes for changing a boars sow. Even after quite few unproductive matings, never assume the sow or boar is sterile, particularly the boars.
I, and a friend of mine have both fallen victim to that assumption. We had boars that had not bred since birth and when they were about two years old we put them into pens with lots of sows. Within a few weeks her's had got five pregnant and mine had got three!. They obviously had preferred to 'pick their own,' so to speak! It was either that or the experience of suddenly finding them surrounded by bevies of sows in abundance had worked wonders for their libidos!.