Figure 2b . You may have observed a very weak response by the sperm to the control toothpick. Typically, a very slight response can be observed even for the control toothpick. Since there is also a very clear difference between this background response and the positive chemoattractants, it does not invalidate the experiment. In fact, interpretations of many types of experiments must typically factor in background level responses.
Succinic acid shows a very weak response.
Examine the chemical structures provided by your teacher. All of the test substances are four-carbon acids. Succinic acid lacks the hydroxyl group that is present on the others. Since it gives a weak response, the hydroxyl group appears important for recognition. Presumably, membrane surface receptor sites on the sperm have different affinities for the different chemicals, and this is based on their structure.