SF600 modular cables won't go in all the way

I recently just completed a build using the SF600, and although I got it working it doesn't seem like the modular cables are fitting properly into the PSU. I was able to get the bottom row of cables in (CPU, PCI-E) although with much more difficulty than I was used to with previous Corsair modular PSUs, but the top row was far more difficult. In the end, even with a lot of jiggling and an absurd amount of pressure, I was completely unable to get the ATX cable plugged firmly into the PSU - there's a good 2-3 mm exposed and the retention clip doesn't click into place. Has anyone else encountered this before?