The type or namespace name 'Plugin' does not exist in the namespace 'Nop' (are you missing an assembly reference?)

I know this question has been asked many times before, but I havent seen a proper explanation of why it happens.

After building a plugin then doing the build on nop.web I keep getting these errors, it was working fine for a while now I get it everytime i try to build, even building one of the built in plugins (Payments.Manual) I get the errors.

I can see the configure view is using:
@model Nop.Plugin.Payments.Manual.Models.ConfigurationModel
and the namespace error shows up in the file, but why does it work sometimes and not others?

I have tried clean, rebuild etc but to no avail, im using nop 4.3.

Is it something to do with the .csproj file?

Its very frustrating, I spend most of my time figuring out what the problem is instead of writing any code.

Thanks.

Hi
Close all opened view files in visual studio and then try to rebuild.

I tried that with no luck, Im pretty sure it has to do with the .csproj file in nop.web, when I revert back to the original it solves the problem, If it happens again ill compare the 2 and see what the problem is.

try deleting the Plugins folder under Presentation\Nop.Web folder
and build again