Problem with loading other dll

I have .net project where I run website within I use more then one dll libraries created by MATLAB BUILDER NE. For example I have one part of website where calling function_1.dll created by MATLAB BUILDER NE and another part of website calling function_2.dll created by MATLAB BUILDER NE . But here is the problem. When I start IIS server and run website with function_1.dll everything runs correctly. But when I try to run website with function_2.dll website returns me an error ! I must restart IIS server and then I run website with function_2.dll. Please help me where is the problem ??