Views Contextual Filters: Using node's URL alias as a conditional filter
Maybe the View Url Path Arguments module. I am seeing it recommended in this thread, but I'm really not sure if that's your issue or not.

Alternatively, you can implement hook_views_pre_view and alter the argument passed to the view based on the value in the url, i.e. do the conversion from alias to id manually. If you have an issue when your alias contains a /, an alternative may be to use a query string for the contextual argument.