I teach the front twisting as such
I first and foremost teach a whip front layout (or a tight arch layout). This is mainly because the gymnast will need to learn how to generate rotation from the lower body (as all tumbling rotation is about flipping the lower body and hip over the shoulders) much earlier in their gymnastics then to need to twist.
Once they understand and can perform this layout correctly (which may i add is a total pain to coach because of the head position, and so forth), i move on to teaching front hollow layout.
Initially i teach the whip layout with the arms going sideways, but when the hollow layout is introduced i teach a punch (this of course is important as front twisting layout not matter how done are more often and ideally done using a punch over arm technique)
You can teach this on a tramp, of a springboard first, rod floor. I dont suggest teaching it on floor. (no surprise here)
So the hollow technique of the layout is encouraging a slightly wide down arm swing action (like for a punch front). On the take the gymnast punches and swings the arms down hollowing, followed instantly by a leg drive. The focus here is HEIGHT!. In so the rotation will be slow but that is all good as long as its enough to a little more then required to complete the somersault fully straight. ( of course you want to start getting them to doing this layout onto a raised surface pretty quickly and before teaching the actual twisting, because otherwise you will get a them twisting early, because they would not have learned the set well enough)
Now basiclly it a matter of getting the gymnast to understand an uneven arms swing. The more aggressive and asymmetrical the arm movement the better. The first 1/2 twist should be completed vertically. To try and get this they need to spot the floor after the take off.
I am sorry if this post is of little help, i am finding it hard to write this today haha.. Hope it helps a bit.
Overall poor landing is = to poor take-off... incomplete twist is = to poor take-off (lack of height) and sufficiently fast twist (and thus partly to do the rotation)