In one of our projects we were referencing custom assemblies that were part of the same solution. However those assemblies were not getting copied to the bin directory of the project upon building the project.
We had also set property Copy Local as true for those class library projects.
The reason for this was as we were adding those assemblies in GAC.
“ VS.NET will copy the dll to the bin directory if your system cannot find the dll in the GAC.”
After removing those assemblies from GAC it added those dlls in the bin.
The helpful thread
Hope it helps..