Before you thicken your jus determine if you are expecting the correct consistency. It shouldn't be as thick as gravy and you should be looking for a slightly syrupy sauce that very lightly clings to a spoon.
If you feel your jus does need thickening then most likely you haven't reduced the sauce enough. Therefore, keep it bubbling for a little longer until you have the desired consistency. In a pinch, you can also use cornflour/cornstarch. Use a teaspoon at a time, mixed in double the amount of water first, until you get to the preferred texture. Note that using cornflour/cornstarch might affect the flavour a little.