What's wrong with your practice method.
Have you been studying a piece and you struggle always in the same passages?
Do you make all the effort you can, but the problem is still there?
You will be happy to know that most of the time, the difficulties you have are due to the way you practise and not to actual limits!
Let's see together how you can make your practice more effective.
Lack of time
This is, of course, an obstacle to your progress, so it may be the reason why you are stuck on those pieces.
The time you should spend on the piano depends on your age, level and repertoire, so always ask your teacher what is best for you in order to keep improving.
Apart from the length of the practice-sessions, ideally, we should practise every day. This always makes a huge difference in your results and satisfaction. Try and establish a routine, so that every day you can play the piano. Of course, there will be days in which you are not able to touch any key and days in which you have only a little time, but if you build up good habits, it will pay you off for sure.
Quantity over quality
Especially when you struggle finding time for your beloved instrument, work on the quality rather than the quantity. If you are not focussed and you try to play everything without listening to yourself, you are not practising. You are just pressing keys with no control. No wonder if there is no progress.
In this case, you need to plan your practice giving priority to the difficulties and making sure that overall, during the week, you are able to work on your current repertoire. Try to study every day the most difficult things, e.g. a new piece, a difficult technical passage, the expression of a particular piece, etc. This way, you work on your weaknesses every day and temporarily dedicate less time to the rest of your programme.
In general, set a priority-goal for every session and spend the necessary time on that only.
You just play through
I know, we learn the pieces to perform them. But playing them over and over won't make them better. More likely, it will strengthen the bad habits.
We do need to play our pieces all the way through from time to time to check our preparation and work on the actual performance. But what we need much more is to know our repertoire: the composers, their works and every detail of the piece we are learning. Do some research and then open your book and look at the composition you are studying. There, there are all the answers you need.
Analyse the structure of the piece and work mainly on what needs more practice. Divide the piece into sections and short passages to go over every detail. Ask yourself if you are following exactly what is in the score, if your technical approach is correct, if what you play makes musical sense, if you are taking care of both hands equally, etc. Every day you will find something new to work on. Quite challenging? Yes.
You forgot the music
Sometimes the daily practice can make us unenthusiastic. We just go over the pieces, focussing on our issues mechanically. This can result in unexpressive performances and technical difficulties.
If you work on a passage that is technically challenging, don't forget that you are still playing musical phrases. You should solve the issue respecting the musical meaning of that passage. Listen to yourself carefully and find out what is wrong. For example, if you are working on a passage that involves the end of a phrase and the beginning of a new one, make sure that you "breathe" between the phrases. That natural little break between phrases could be the key to solve the issue. In fact, your breath will improve the expression of the passage and will make you use the right gesture.
This is just a little example, but, in general, playing without expression makes the virtuosity even more difficult to manage. So, feel the music you play and sing it, especially when it gets tough.
You are super-tense
This is probably the most common problem. You are so completely involved in what you are playing that you cannot realise how tense your muscles get.
To solve this problem, I recommend some relaxation exercises that help you coordinate simple gestures with your breath. Breathing is essential to maintain your mind focussed and your body relaxed. If you include these exercises in your practice routine, you will use your body better in your performances and also, you will become more aware of your body, which is the first step to control it.
In simple words, every unnecessary tension affects the way you play, so the key to success is efficiency: reduce the movement to the basics and you will progress quickly in terms of quality of the sound, technique and endurance.