(Some spoilers ahead)
The reason why the other members of Avengers could not come to help Captain America and Black Widow is well explained in Marvel's Agents of SHIELD TV series.
1) Compartmentalization of SHIELD that prevents information from disseminating
2) Level 6 Agents that are the Avengers do not have access to information for Level 7 agents and higher
3) Hydra prevents information from getting to the other Avengers
4) The other avengers and other agents are too far away from walking distance to be contacted
The other avengers are not aware of what is happening at SHIELD because of the compartmentalization of the organization. So, level 6 agents that are the Avengers cannot know things that are happening on a higher level 7 and above(Agent Coulson, Maria Hill).
It is specifically because of the compartmentalization of SHIELD that is why information can be slow to reach the other Avengers(level 6 clearance) until it is too late. If there were no compartmentalization, information would be instantly shared with all agents, and all members of Avengers would have total access to all information at SHIELD and they would be able to instantly react and arrive to help Captain America and Natasha instead of being kept in the dark.
Director Fury got shot because he tried to contact Captain America. This also proves that Hydra was preventing Fury from seeking help from members of the Avengers.