When there are about ten cards, large ones stop fitting on the screen: the bottom row goes past the edge, and the learner does not see it — which means they cannot choose from it either.
The main fix is to make the cards smaller so that the whole choice is visible at once: three dots → “Settings” → “Card size and spacing” → “Customize”. Move the “Card size” slider until all of them fit on the screen, and tap “Save”. Size belongs to this device: a phone, a tablet and a computer each keep their own.
If the learner needs large cards — poor eyesight, or the finger often misses — keep them large and turn on “Scroll cards by screen” in the same place. The cards then scroll a whole screen at a time and are not cut off at the edge.
Do not make the cards smaller than needed: the learner should hit them confidently with a finger. If ten do not fit even when small, put some of them into folders — see the “Categories” lesson.