Important question: what do you mean by 67% accuracy? That is, are you talking about the training accuracy or the accuracy in a separate validation/test set?
If it’s the training accuracy, then there’s either some problem with your implementation (bad learning rate or even a bug in the neural net definition) or your model is too small. In both cases, this does not mean that a CNN is not capable of solving your problem.
If you’re having a high training accuracy (> 90%) but your validation/test accuracy is only 67%, then you are overfitting your data. This could happen if your training set is too small or if there’s no relevant structure in your data that the CNN can capture to classify the waveforms.