Given what Geocom said about the data lines not being common, it would might matter if you put the strips in backwards. Judging by the backing on the double sided tape, the strips may not all be the same way around. 

Edit: Actually, judging by the wire colours, they are the same way around, so above suggestion is irrelevant.

@richms nailed it. I tried flipping the data wire over on to the front pads and that solved the problem.

Until...

I bought LED strips that had a silicon cover on the front. Project is destined for outdoors and while the project box will be weather sealed I figured it wouldn't hurt to add the silicon IP65 rating. It didn't cost any more. But it did mean that to solder on the front pads I needed to carefully cut away the silicon cover. Operative word being carefully. Given the first four had gone well, I got a bit careless and managed to slice right through the whole strip on number 5 and lost an LED. I've declared the proof of concept a success anyway and ordered a bunch more for the actual build. Without the silicon cover!