There are some positions your wife can adopt after sex that supposedly improve the chances of conception. I have no idea if they really make that much difference.
It can just take time for this to happen, but if you are worried I would suggest both of you see a doctor to have your fertility tested.
Finally (and please don't take this the wrong way, I know absolutely nothing about you, your wife or your circumstances), it is possible your wife does not want a baby quite yet, but can't bring herself to tell you, and has been taking The Pill without you knowing. It wouldn't be the first time in history.
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