Why does my roast beef smell wonderful but tastes bland?
If we're talking about a solid, four pound cut of beef - the only flavor you're ever really going to get is on the exterior and just a little bit into the interior of the meat.  

That said, cooking in the spices/components you list still may provide liquid gold.  I would simply take some of the liquid that's leftover in the slow cooker after the roast has cooked and make a gravy out of it.  You'll get the flavors you put in and have something to sauce the interior of the meat - that have been flavored with the juices of the meat as well. 

Otherwise, if you're looking to 'infuse' more flavor into the beef itself, you'll need to consider something like a stew rather than a whole cut.