Using an empty bar allows you to work through slightly more range of motion than an ab wheel. The bar sits a little lower to the ground which means your body can fall flatter.
This will also place the points of contact (the bar and your knees, or feet if you're doing them standing) further apart. The further the distance between these contact points, the harder your abs must work to keep your lumbo-pelvic region positioned correctly.
Dr. Ken Kinakin, much like Dr. Mike Leahy, knows your pain. In fact, he's made a living off of it. The doc is one of the best in the world at devising training protocols to help make you injury-proof. Yeah, he likes bodybuilding and powerlifting routines, too, but when it comes to figuring out how to fix what ails ya', he's one of the best.
We all go through periods when pumps elude us. Frankly, these periods really suck. And worse, they seem beyond explanation. It's a bloody shame, considering how motivating enlarged, vascular muscles can be.