My first attempt would to try analysis the problem and resolve them step by step

☛ Firstly I would try to see whether it is a new project, and then like to re-check the key words.

☛ Also, I would look for relevant key-words that can be helpful.

☛ Even though the webpage and website has been indexed well and still not appearing on the first 10 pages of search engine result page, then I would make some changes in page text, titles and description.

☛ If website is not indexed well or dropped from the index, than it might comprises serious issues and re-work might be required.