Vishal Mega Mart — I have 500 rs vouchers but don't accept in our branch

Address:

600017

I am purchase in our branch in t.nagar above 4000.i got gift voucher for 500 rs.i have valid period for one month. i change this voucher in tirunelveli district unlimited mart . i dont accept that voucher. why . i have points for 300rs they less that point but in cash say voucher dont accept in this branch