The syrup is runny when there is relatively more water in the syrup. The simple syrup has a 1:1 ratio of sugar and water. But if you are aiming to make a syrup for bubble tea, you might need a thicker syrup with a 2:1 sugar to water ratio. There are two ways to fix a runny syrup: One, by cooking more: By cooking the syrup, we evaporate the excess water, and the syrup will become thick eventually. This method is right for you if your syrup needs a very little adjustment. Cooking for 3-4 more minutes might fix it. But if the syrup is very runny, like simple syrup and you want to make it into a boba syrup, cooking is going to be a wasteful exercise. Sheer wastage of time and energy. I suggest you try the second method. Second, by adding more sugar. Say for example you made a simple syrup and now you want to use it to make tiger milk tea. Your only solution is to add more sugar to make the syrup thick.