Meal tickets / lunch vouchers refused

What is the deal with meal tickets / lunch vouchers ('Chèque-Repas") in Belgium? Ever since I started receiving them I've been told they are legal tender or at the very least accepted in any outlet that sells food. Now for the second time in a week my lunch vouchers have been turned down - first in an Irish bar in downtown Brussels, then in Stonemanor, the British shop in Tervuren.
Is it at the manager's discretion?