It can be difficult to understand a foreign language even with a decent knowledge of the language they are speaking. But sometimes knowledge only goes so far. Dialects, age and maturity can all affect the way someone says a word and this is true across all languages.
A good way to get around understanding what people say, even if you miss a few words is to take note of their body language. If you don’t already do this then it can be hard to force your brain into noticing how someone says something, as well as what they are saying. But as you notice their way of speaking, through their body language, then like learning to swim your brain will seem to accept it into its repertoire of skills.
Slowly you will start to learn commonalities in the way people say something which will help you to discern what they are saying and your brain will automatically fill in the gaps you might of missed.