Was the tool you were using "exactly" as in the original picture?
It isn't the W-HT-1884 (which looks identical)?
The proper tool is 11-03-0016. The W-HT-1884 is a hair wider than the 11-03-0016 -- not much but enough to make it difficult to use as it doesn't fit into the end slot properly. The 11-03-0016 fits perfectly - but, of course, costs considerably more.
I've also found out that if you push slightly on the wire into the plug while inserting the tool also helps - it removes some of the pressure of the tang hitting the back side of the retention slot. After doing a few of these, you tend to get the hang of it and it gets easier...unless there is dirt/corrosion/other crud within the contact area.
Ed