A coupon fails to apply if any of these are true: it's inactive, expired, or has no uses ("remains") left; it's restricted to a module you don't have anything from in your cart ("not available for these ordering systems"); it's a one-per-customer code you've already used; it's restricted to a specific customer or membership plan that isn't yours; it's an "only on your first checkout" coupon and you've already checked out before; or it can't be combined and you already have another non-combinable coupon applied. Separately, as your cart total changes, a coupon already on your cart (or its tiered discount rule) can also be dropped if your subtotal falls below its minimum spend requirement.