Discount not applied

I have a variable product (options are right or left handed). I’ve applied a quantity of 2 and a discount of $10 – so if someone orders two, they get $10 off – yes?
But it doesn’t work. The cart shows the price each then the same price with a line through it. Like it’s trying to work, but hasn’t taken off the discount. In the total column it says:
$60 (With $ discount)
Like it knows there’s a discount but doesn’t know the amount. I’ve double checked the amount – I’ve definitely added a 10 in there.

Thanks
Bob