EntryPointNotFoundException - but entry point exists

I have a C/C++ unmanaged DLL that is working with the exception of a single method which causes an

EntryPointNotFoundException

i'm setting up the DLL in c#

Code (CSharp):

1. private const string dllname = "mydllname";
2.         [DllImport(dllname)]
3.         private static extern IntPtr CreateBridge();
4.  
5.         [DllImport(dllname)]
6.         private static extern void DestroyBridge(IntPtr context);
7.  
8.         [DllImport(dllname)]
9.         private static extern int getChannels(IntPtr context);
10.  
11.         [DllImport(dllname)]
12.         private static extern void setUpdate(IntPtr context, IntPtr values);

and then calling the method

Code (CSharp):

1.  
2.                 IntPtr updatePtr = Marshal.AllocHGlobal(Marshal.SizeOf(update));
3.                 Marshal.StructureToPtr(update, updatePtr, true);
4.                 setUpdate(context, updatePtr);

Before I'm calling

setUpdate(context, updatePtr)

I'm successfully calling

CreateBridge() 

and

getChannels(IntPtr context)

so we know the DLL is present, that it is loaded and that other entryPoints can be called

But

setUpdate(context, updatePtr)

causes the exception

EntryPointNotFoundException

the C definition is as follows

Code (C):

1.  
2. #define DLL_EXPORT __declspec(dllexport)
3.  
4. ...
5.  
6. extern "C" {
7.     DLL_EXPORT  UnityBridge* CreateBridge();
8.     DLL_EXPORT void DestroyBridge(const UnityBridge* _unitybridge);
9.     DLL_EXPORT int getChannels(UnityBridge* _unitybridge);
10.     DLL_EXPORT void setUpdate(UnityBridge* _unitybridge, Update* update);
11. }

What else can cause this exception?