It looks a little better this morning but yes, still alittle darker than what was under the sofa which has been there since the carpet was new. I do think I used too much pre-spray or too wet pads since it took all night to dry. I didn't know which. The vibration may be happening because of not centering the glider enough? Do OP machines tend to pull to the left?

OP machines have some tendency to move with the run of the pile, or of the loops. Some OPs do this more than others. The first full size OP machine I used was the Oreck LowBoy, later renamed the LB-100 or something like that. It has a bristle type of pad holder, which won't hold a glider in place. When I decided to buy my first Challenger specifically because of this weakness in the Oreck, I didn't know what to expect. When it arrived, I was very pleased to find that any pulling of this nature was greatly reduced due to the ability to use a glider.

There are several reasons why berber carpet will never look the same in heavily used areas as it does off the beaten path. Some of this may be due to the tightly looped fibers limited ability to allow the cleaner to scrub away all of the soil. But there are at least two other reasons, which you may sometimes have to explain to a customer.
One is that when heavy use occurs, the fibers themselves become abraded, with some or all of the shine or sheen produced by the manufacturer being worn away by foot traffic. They develop a dull appearance which can be confused with being still soiled. The other is that the fibers get packed down compared to the fibers in other parts of the same room. Since they are now standing - or leaning - at a different angle than their less beat up counterparts, they are going to reflect light at a different angle. The eye likes the look of the original straight up, unabraided fibers much better that those which have become dull and beaten down.

After some dry time it looks much better. I am sure I overwet the carpet as it took overnight to dry. Not sure whether I put too much prespray down or the pads were too wet. I did prespray 3 times though as I used a pad 1st, then fiberplussed it, then used a pad again to get the soil up. Do you run into this if the stain is too dark? I used Orban. Should I have just kept running the Challenger over and over the bad part more instead? I want to do the rest of the carpet today and there are a couple of more bad stains, not as bad as the one I got up but bad anyway.

Orban?
If you have the time and inclination to do so, you can continue to go over a carpet area of any type until you see no more soil appearing on your pad. But remember to keep the machine moving. Don't let it just keep running in one area without moving it back & forth or from side to side.