To clarify, the čárka in Czech results in an elongation of the vowel sound. If you are listening for some kind of stress, you won't hear it. The amount of length increase varies by region. As I recall, in Ostrava people practically ignore them when speaking.
Is there a difference in pronunciation between "potrebuji" (I, first person singular) and "potrebuji with the carka" (they, 3rd person plural)? I put it without the carka and it was counted correct. But the translation is "they".