Pathfinder Unite not uniting everything

Hi Experts

My attempt to use Pathfinder to Unite objects is not completely uniting them.

Here is the artwork. Basically a frequency distribution with each bar

I want add a stroke to the distribution but only to the outside edge, not to the vertical lines for each bar.

So I thought that Pathfinder Unite would do that.

With all the bars selected like above I ran Pathfinder Unite and got this:

It is uniting most, but not as one single object.

Any ideas what might be causing this?

Thanks!

jballou