Yes I have cracked the bolts loose 
No matter what i do, I can’t get the low profile vise to secure both parallels down. It only ever clamps the parallel down on the moving vise side. I’ve tried with 123 block and numerous pieces of stock, large and small.
Any thoughts?
If you loosen those bolts more, you’ll eventually find success with a true 1-2-3 block. The vises operate on simple fundamental wedge physics.
Note that though perfectly normal, clamping a part in a vise like this is overconstrained. I find that unless my part is extremely square, I often only get it fully clamped down onto one parallel. If you jiggle the parallel, you can feel how much vertical play there still is.
Are you tapping the part down with a soft hammer? I dont believe there is such thing as a vice that will consistently pull down a part against both parallels 100 percent of the time. Thats my experience with Kurt and Orange, at least. The “sliding jaw” thing really just helps with retention, IMHO.
Like TurkeyLeg suggested, a few taps with a rubber mallet is my go to. You don’t have to beat on the parts generally, just some mediocre taps as I tighten the vise is all it generally takes.