These masks that are proving unsuitable should be returned under the sale of goods act as not suitable for purpose.
They should not be paid for until they have been proven to be up to standards required and any adjustments should be at the cost of the supplier.
If wording similar to this is not in the contract then the contract should not be issued to the supplier.
How many of you guys with your own businesses would sign a contract with a supplier, and pay up front, for goods that were not fit for purpose?
You would at least expect your full money back.