Why doesn't my caramel sauce thicken?
Sugar "boils" at a temperature much higher than water. The water is there just to get things started, so you don't burn the non-melted sugar directly in contact with the pan at. Within a few minute the water will have evaporated, and all you have left is liquid sugar

Use a candy thermometer and slowly raise the temperature until you get the sugar colour and flavour you want

When it cools it will thicken, or become solid, depending on how high you heated it. It is not just a time thing, it is the maximum temperature your reached

Caramelisation with happen around 170°C (340°F) depending on what type of sugar you use. Caramel sauce usually requires the addition of a fat like cream