General:
* Impediment: Something that hinders or delays progress.
* Hindrance: Something that makes it difficult to do something.
* Blockage: Something that completely prevents movement or passage.
* Barrier: Something that separates or prevents access.
* Obstacle: Something that stands in the way of progress.
* Interruption: Something that breaks the flow of something.
More specific:
* Bottleneck: A narrow point that slows down the flow of traffic.
* Stalemate: A situation where neither side can make progress.
* Impasse: A situation where progress is impossible.
* Roadblock: A physical barrier that blocks traffic.
* Logjam: A situation where there is a build-up of things.
The best synonym to use will depend on the specific situation.