If you simmered the simple syrup for a long time, and ended up with a very thick and rich simple syrup - you run the risk of the sugar crystallizing in the syrup. The higher the sugar concentration in the syrup, the higher the risk of sugar crystals forming. A rich simple syrup may crystallize more than a simple syrup. You can add additives such as corn syrup or cream of tartar to prevent crystallization. But making sure not to let the simple syrup simmer for too long, and letting it cool completely will also prevent this from happening.