dwm top bar does not update if attached to external monitor

[xormapmap]

I have a fairly confusing issue. I've set up an old laptop with Gentoo, which I plan to use as a desktop. I'd like to eventually just put the laptop in my desk drawer hooked up to an external monitor and a mouse and keyboard.
For that reason I'm trying to get dwm to work with an external monitor. However this is what currently happens:

Boot
After a little while I see the boot output on the external and normal monitor
Then I see the login prompt on the external and normal monitor
I log in and hit startx
dwm loads, external monitor mirrors main monitor (exactly what I want)
But now in dwm I hit Alt+2 to change desktop and it looks like nothing is updated (on either monitor).

What I mean by this is that the highlighted tag in the top left corner does not change. I can also see the output of my xsetroot text in the top right corner is truncated.
However, my windows do change. If I have a terminal open on desktop 3 and I press Alt+3 I do see that terminal. I do see its output reflected on both screens.

The odd thing is if I do this exact process with the external monitor unplugged then dwm works flawlessly on the built-in monitor.

It seems that having my external monitor connected breaks the top bar on dwm on both monitors.
All I really want is my external monitor to mirror the built-in one with dwm working.

I'd post the output of various configs/logs but as I'm fairly new to gentoo I don't even know where to start on this issue. Let me know what you need and I'll get it to you when I can.

Cheers.

[JohnJanzen]

Have you set the monitors to the same area with xrandr ?
Are they maybe different resolutions ?