Dependency not injected although service exists

So I've been working on an Angular.js project for awhile now that will be used in a competition, and halfway through I realized I needed a plugin system to dynamically load some necessary code at runtime. Thus far i've found that angular.js isn't very accommodating to loading services at runtime. Due to the code only being available to competitors at competition time I can only provide an example of what isn't working.

// ex1.coffee

mainModule = angular.module('mainModule')

mainModule.service('needed', () ->
    @neededFunc() ->
        console.log "just an example"
    return this    

mainModule.service('ex1', ($injector) ->
    @loadFile = (u, pluginName) ->
        $.ajax
            dataType: "script",
            url: u,
            data: null,
            success: (jqxhr, textStatus) =>
                $injector.invoke([pluginName, (plugin) =>
                    plugin.testFunc()
            error: (jqxhr, textStatus, errorThrown) ->
                console.log "couldn't find the script"
    return this


// ex2.coffee

mainModule = angular.module('mainModule')

mainModule.service('ex2', (needed) ->
    @testFunc = () ->
        needed.neededFunc()
        console.log "in this dynamically loaded file"
    return this  

Implicit things: There is an existing module called mainModule. Jquery and angular have been previously included in the dom. ex1.js is already included in the dom while ex2.js has not yet been included in the dom. u is a url to the file ex2.js and pluginName = "ex2" . When the success handler is called, the invoke function fails, saying that an ex2Provider does not exist. However, if I breakpoint in the success callback and check the mainModule._invokeQueue, there does exist an array with the service name "ex2" so it definitely has an existing recipe within the module, but the $injector.invoke call still fails. If I modify the success handler to call, $injector.has pluginName, it fails. Can someone tell me how the module can have knowledge of a provider for this dynamically loaded service but not be able to inject it into a function call with $injector? The project is a yeoman project built with grunt.

Still learning angular so if there is some concept I'm missing or stating wrong, please tell me.