This is a common question in aikido. There are several answers. First, ask the instructor. Most likely there is something you are doing incorrectly.
Second, aikido techniques, as we practice them in the dojo, are idealizations. No aikido technique works all the time. Rather, aikido techniques are meant to be sensitive to the specific conditions of an attack. However, since it is often too difficult to cover all the possible condition-dependent variations for a technique, we adopt a general type of attack and learn to respond to it. At more advanced levels of training we may try to see how generalized strategies may be applied to more specific cases.
Third, aikido techniques often take a while to learn to perform correctly. Ask your partner to offer less resistance until you have learned to perform the technique a little better.
Fourth, many aikido techniques cannot be performed effectively without the concomitant application of atemi (a strike delivered to the attacker for the purpose of facilitating the subsequent application of the technique). For safety’s sake, atemi is often omitted during practice. Again, ask for your partner’s cooperation.