Some wheels just cannot be trued.
Wheel truing is working in three dimensions- true/straight, round and dish. A rim bent beyond the limits of
"mostly straight" will never true up correctly.
The easiest way to assess the situation as to whether a wheel/rim is true-able is to release the tension by loosening the
spokes of the entire wheel. Done evenly, round an around the wheel to the point where the spokes don't rattle, but there
is little tension, and spinning the wheel.
A "tacoed" rim will be easy to spot.
In general, spoke tension can be increased/decreased to facilitate true, but each turn of a spoke nippel affects all three.
The issue is that a bent rim acts as a spring. Once a rim is sprung/pretzeled/twisted/tacoed, it will try to return to that state.
Methods are varied. Basically, its a process, and a little turn of a group of spokes does more to correct the imperfection
than turning one spoke a lot
A final seating of the spokes after truing a wheel can "release" the spring. I do this by laying my forearms across the wheel,
with my hands at 10 and 2 o'clock and flexing the wheel a bit.
Doing this, you will hear the spoke ping(set). I turn the wheel 90 degrees and repeat all around the wheel and then check
the wheel for true , round and dish once again.
A wheel that does not hold basic true after this act, likely never be ridable to any degree..
When the spoke tension in one section of the wheel is to much different than another, you have a very unstable situation.
The slighted bump and wham- the pretzeled rim returns.
Hope this helps a bit. It takes a while to be good at wheels, but well worth the time involved.
rusty