Need Help with Cinnamon Roll Filling

[SzczerbiakManiac]

I've been experimenting with baking cinnamon rolls as of late. I'm very happy with the taste (especially the frosting), but my cinnamon filling keeps crystallizing and making "candy" at the bottom of the roll. I don't want that. I want the filling to stay gooey.

Right now I'm coating the inside of the dough with a thin layer of margarine and then sprinkling a mixture of dark brown sugar and cinnamon on top of it before I roll it up.

What am I doing wrong? Do/can I need to add something to the filling to keep it soft?