Are you saying that you have SQL server database instances on a cluster and a particular application that uses them only works when it's database instance is running on one of the nodes of the cluster but not the other?

Or are you saying that one of the databse instances only works on one node and not the other?

When the database instance fails over to the other node does it come up successfully? Are you able to connect to the database via management studio when it is on the other node? Are you using static ports or dynamic? Is the browser running on both nodes? Is the Windows firewall blocking the port on the other node?

Sorry, a lot of questions but it will help to understand what is happening.

If it is applications running on the cluster rather than database instances, you may have to contact the vendor for support.