I've been painting a floor cloth and all was going well until I tried to attach painter's tape to an already painted stripe(dry). I can't seem to get it to stick any suggestions?

Painter's tape does not stick well to cloth.

If you can get the painter's tape to stick to the extreme ends outside of the cloth, like the surface on which the cloth is lying, I would then use the tape like a straight edge. You will need to hold the tape down with your thumb and index finger beside where you are painting and then keep moving your thumb and index finger along as you paint.

This may sound wild but try a test piece with a little rubber contact cement. The rubber cement gives you a crisp line that you paint cannot seep under. When you are done painting and the paint has dried you take a white eraser and rub the rubber cement off. Due to the nature of your substrate I am not sure about how safe the contact cement is though. Try a small swatch of the same material to check the absorption and removal of the contact cement.