Stamps on cloth are tricky.
First of all, size of stamps should be larger to fit to the size of bag.
Secondly, the stamp pad colour, no matter how thick of layer I apply to the stamp, appears quite weakly on cloth. The cloth texture and its absorbing ability of ink are not easy to estimate.
Stamping on cloth takes more energy, seriously.
Something I need to work on a little more next time.
DIY Eraser Stamp 
Helen, have you tried using acrylic paint? I simply “painted” it on to the stamp, making sure it was even.
Thanks for your advice Carole. I saw people using acrylic paint on cloth and it looks nice. I wonder if I can apply it on the stamps and stamp them on cloth?