Let's say he starts the tests with the one legged stand. Have you tried this at home? It isn't easy even in the best of conditions. You are asked to stand on one foot for 30 seconds and keep your hands at your sides. There is the first problem, keeping your hands at your sides while standing on one foot is not the way humans are supposed to stand. Have you ever seen a baby take his or her first few steps? Their arms are out to give them balance. Granted you aren't a baby but, you don't spend your waking hours standing on one foot so yes, you are going to fail.
There are multiple problems with the one-legged stand. Frequently, when some one is pulled over for drinking and driving; it is at night. Is the ground you are standing on level? Its dark, how can you or the officer tell if you are standing on a flat surface? They can't and now you are at a disadvantage. Age and weight come into play during the one-legged test. Older people are not always steady on two feet let alone one. Being overweight can cause problems with balance.
Most of the time these test are administered by the side of the road which creates a very stressful situation. First of all, cars are buzzing by creating a distraction. If an 18 wheeler flies by the draft alone could cause you to lose your balance. You may be nervous. You wonder if anyone you know is watching or what will happen if you have to spend the night in jail. Those circumstances alone will make anyone wobbly.
If you are arrested solely on the grounds that you failed the Field Sobriety Tests, you have the right to contest the findings. A knowledgeable attorney is your best defense.