Since I decided to wait for the old brakes that were on the bike to wear down, I've only now got around to fitting the Clarks brakes. Because I was putting new pads on I decided to get new v-brake calipers as well. The old v-brakes (apparently) are Tektro C310 V Brakes, so I went and bought a new set of Shimano Alivio BR-T4000 V-Brakes. I tried to fit these this evening and couldn't.

To best descibe the problem, please see the following images - these are NOT my pictures but they show the exact same problem I encountered tonight. The Shimano brakes don't sit flush against the frame connector. The small tension pin doesn't fit fully into the hole. There seems to be a boss on the frame lug that prevents it seating fully. Am I missing something obvious??

Thanks in advance.