Siri does a pretty good job of learning your voice and dialect so it understands you. However, if you've found that it's not doing a its job, Mac Observer points out that it's easy to reset and start anew.
All you need to do is turn Siri off and back on again. Head to Settings > General > Siri and toggle the Siri option to Off. Then, head to Settings > General > Keyboards and turn off "Enable Dictation." After that, go ahead and toggle both of those back to on and you'll have a clean slate with Siri.
iOS: Deleting Siri's Training Data [Mac Observer]