Sometimes it is difficult to hear opposite persons Japanese and catch the meaning of it at a point. It is good to ask the speaker politely to say it again. This is how you can ask for it:
Japanese: もう いちど いって ください。
Reading: moo ichido itte kudasai.
Meaning: Please say it again.
If you still find it hard to figure out the speakers word, you can try asking:
Japanese: ゆっくり はなして ください。
Reading: yukkuri hanashite kudasai.
Meaning: Please can you speak a bit slowly.
At this point if you still unable to understand the speakers word, it is better that you call someone who has better Japanese understanding and help you right away. But if the topic is not that much serious it is always nice that you try by yourself to solve the issue. In this way it will improve your Japanese in time. Good luck!