Feedback from Tumblr Support regarding the missing REBLOG button
The typical source of that problem is browser-add ons that users have installed on their browser. It can sometimes override the iFrame that contains that button. Considering that we just made a few changes that button, it’s especially possible. A good way to test that is to go to a post’s permalink page and check if you can see the iFrame/reblog button. For example, I can see yours fine here: http://banksystreetart.tumblr.com/post/32880061860/i-have-contacted-the-fine-folk-at-tumblr-support-to. If you can’t see it on your end, then it’s more likely to be a coding problem. You’re right, you’d probably see a lot fewer notes if this were the case.
If you can’t see the reblog button in the iFrame (that’s the little frame with the buttons on the top right of your screen) than it’s assumable that the problem is related to your browser or any browser addons.