coupon code not deduct the cost

hi..i’m using plugin version 6.00.31 and wp 3.7.1
i’ve enable coupon code to give some discount.
The problem is..the coupon code did not deduct the cost,
for example;
my total cost is 100 and coupon code (discount) is 30
total cost should be 70 after discount but after i put the coupon code, total cost increase to 130
please help

http://wordpress.org/plugins/event-registration/