Your comment hints at another problem, which is that allowing the cargo into a port possibly could be exploited as a loophole to break sanctions.

Yet another big problem is that cargo might be too low in value, or even undesirable. Like the cargo of Ammonium Nitrate that exploded in Beirut a few years ago (it had been taken off the docked ship which then sank in the port. The cargo was stored in the port, then stuck in legal and payment disputes, and the result was horrific).