It is a "tight fit", but once you get it on there, it is all good. You might have your cables swapped.
Here is a pic. of Tuna's. I had to trim off a corner of the plastic(top right of this pic) in order to get the cable to stretch HARD into the holder at top right.
The trimming would look like an "arrow tip" angled/pointing inward between the two cables, (again, top right of pic.)except for I think I only had to do this to the top cable side, instead of both. The trimming was very slight, as I did not want to remove too much.
Again, I had to pull the CRAP out of it to get it to finally slip into the retainer.