Some seem to have run into a problem understanding what straightness truly is.
In a nutshell it is the even loading of carrying power and thrust on each leg so that the horse is able to do its job equally on either side whether it is on a bend (circle/arc or) or on a straight line.
In longer terminology I had posted this.........
The concept of crookedness/straightness is complicated so I will not try to get you confused with a lot of terminology but suffice to say that crookedness is viewable in both the horse and rider.
Crookedness is caused when the horse's hind legs are not working with equal thrust and support. If for example the horse is traveling left and the left hind does not stride forward as far as the right then it does not supporting the shoulder on that side. This means the left front leg has too much weight and the shoulder leans into the left shoulder with the horse bending to the outside or to the right. This will force the right hind farther out to the right so that it no longer remains under the horse and now is supporting absolutely nothing as it is no longer thrusting or carrying.
So how to fix.
The area to look at is not so much the left hind as it is the right hind that is not doing any work. This side needs to be pushed over to bring the right leg more under the body and make it start carrying weight and using thrust so that the two hind legs are working more in unison. The horse may try to avoid this extra work by pushing the shoulders left so the rider will need to use their left calf and supporting the neck/wither connection with the left rein. A heavier right seat bone will help also.
Now in all this be careful that the inside shoulder of the horse has not caused you the rider to feel like you are slipping to the outside so you automatically weigh the inside more in an attempt to balance yourself (that dreaded collapsed hip). This will only cause the problem to increase in its magnitude. This may have started out with a minor crookedness on the part of the horse but incorrect riding has cause the problem to worsen.
To do this you will have to be strong and persistent and have the right tools to enable the corrections.
Without relaxation leading into straightness, the horse cannot achieve suppleness or collection and become "through" in dressage.
So for those that wish to know a bit more about the building blocks of dressage, the fact that they need to be built and developed from each other I have started this thread so everyone can have an input.