Loosen in warp direction:
1. Insert wood wedge between nail and weft bar
Loosen in weft direction:
1. Insert chopstick into weft bar between pin and horizontal bar.
If you still can't manage to stretch in weft direction, last resort is putting nakabari, translated as inside stretch bar. Please refer to the pictures.
Make sure to stretch the fabric in weft direction by inserting chopstick into weft bars.
Cut the two pieces of wood so that it will be 1 cm longer than the space measurement between horizontal bar to the other. Or get two adjustable bars as seen on one of the picture.
Place the nakabari in angle so that it will be against to one another. If they get loosen, you can insert strip of wood or make the adjustable bar longer.