I learned Tajweed but when i recite QURAN my teacher says i am not that good. What should i do?

Question
 assalam u alaikum . I learned Tajweed but when i recite QURAN my teacher says i am not that good. What should i do?

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh.  May Allah give you the best recitation and be pleased with you.  Ameen. Make lots of dua' in sujood and especially the last third of the night.
First, listen to the comments your teacher gives you about your recitation and what is incorrect and how to fix it. 
Second practice it, plus listen to known masters of tajweed such as Sheikh Al Husary and Sheikh Abdullah Basfar.
Third, write down the mistakes your teacher says you have and work on them one by one, recording yourself and listening to the recitation of one of the masters reading the aayah, then listen to your recording. 
Fourth, have another teacher listen to you if you still feel you haven't improved after a few months.  Sometimes one teacher has a different technique which helps you. 
Most important, don't give up and don't feel frustrated.  The goal to read the Qur'an correctly is a lofty goal and we may be tested on the way.