Certain types of food consistencies need a little prep before testing.
The sample does not need to be round. We say “pea-sized” to help give a visual of the size of the sample, but it does not need to be round. There is no need to roll it into a ball. Just keep the size small, so it fits easily in the capsule. Too much food can actually cause the test to fail, so it's important not to overfill the test capsule.
To help prevent errors, you can crush or cook hard foods (nuts, frozen peas), rip or chop dense foods (well-cooked meat) and emulsify or dilute powdery foods (dry spices), creamy (chowder), thick (think sauces) and high fat foods (yogurt, frosting).
Puffed foods, specifically puffed cereal or puffed rice, should be crushed. You may need to crush multiple pieces of this type of food to ensure the sample meets the recommended pea-sized. This this type of food is full of air, crushing only a small amount may not yield a large enough sample in this specific case.
Here is a short video to help you understand the recommended sample size.
The disposable capsules each have a small chamber in them which will hold just enough (about 1/2 to 1/4 tsp), and you don't have to completely fill the chamber for the test to work. Be careful not to put too much food in or the test might error. Here are some examples: