What are you doing to WARM UP your horse before you begin pattern work?
Some horses need a long warm-up before they can actually focus on what they are doing. My horse Red always gets a minimum of 1 to 2 miles of normal riding to get some of the silly out of him so that he can focus on the job at hand. And certainly warming up with various exercises (spirals, serpentines, perfect circles, etc) can get your horse in tune to your body and legs.
Also, make sure you aren't nagging on the horse too much when asking them to walk in a straight line. And what I mean by that, is don't try to correct every little thing or you may start to over-correct and cause them to be squirrley on the straight. Think of a car: You drive smoother and straighter if you typically leave the steering wheel alone and only correct with small, smooth movements when you need it. If you constantly moved the wheel to try to stay perfectly between the lines, you'd eventually start over-correcting your driving and wouuld start swerving. Does that make sense?
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.