You have come home from your piano lesson and are practising the new pieces and scales. However, the results are not as good as you would have hoped. Now it's time to look more in detail what should happen during your practice process. 

As with any new skill (or older skill as well), repetition is the key to success. However, if you just repeat endlessly without thinking, hardly any good is being done. If you are already at the stage when you can play piece through, you can quite easily identify the places where you stop or slow down and concentrate your energy on that. The next step is to make the "difficult place" easy, by slowing down the practice tempo, practising hands separately, playing only some key notes from the left hand with right hand and vice versa.

If this by itself doesn't solve the problem, you should give it a few days. Your brain will learn by itself when you sleep, so rather than getting stuck with it, try it again the next day and you will definitely find it if not completely solved, at least easier than before. If you work systematically over several days, the problem will most definitely be solved unless you are playing a piece way too difficult for your level. And that is a completely different subject in itself!