I don't use these from Tayda but something similar that I get off Amazon. (Probably the same mfgr.) As Bricksy said, the genuine Lumbergs will also not seat completely sometimes. This has to do with how tight you tighten the internal nut. When you tighten it, the plastic sheath will bulge slightly not allowing the plug to seat all the way until it's been inserted a few times. The alternative though is to leave it loose enough for the plug to seat but then the jack has a tendency to spin when you plug and unplug the power. I hate when that happens, so I just crank mine down pretty tight.
Funny you mention that. After using the Tayda jacks for years (yes, they are tight, which IMO is good) I bought a few Lumbergs. My 2.1mm plugs slip in/out of those way too easily.