Hi Leena,
I find it really hard whenever a student of mine fails to succeed. I too wonder if it's me or them. Where does the problem lie.
In some cases, it is me. Put simply, I'm just not the right teacher for them at the moment. Other times, it's them. They may not be interested in English, fail to study or do homework, etc.
I've written an article on how to present the target language, which may help. I've also written two additional articles, one on fluency and one on accuracy. These may also help.
Accuracy Fluency How to Present the Target Language
Lastly, have you considered using the L1 of the students? That may help, as they may be able to make connections unavailable in English. In addition, have you considered their learning styles? Some students are auditory learners, other visual, others kinesthetic... tapping into their preferred learning style may help with more difficult grammar/vocabulary.
Hope all of this helps.