Do you find only a few publications that are about your subject? There can be several reasons for this:
One reason for you not getting enough results is that you are not using the right search terms, or too few synonyms. Are there other words that can better describe your subject? Ask your teacher, or try to find a book or review article about your subject so that you can collect more search terms.
Don't forget that you can add many synonyms in the same search with the help of OR, it makes a big difference! Also think carefully about which words that are suitable to truncate, it is often more than you think.
It may also be the case that your research question is very narrow, or that not many people have done research on your specific subject of interest. In these cases you may need to broaden your search (and perhaps even your research question). Perhaps it is not necessary that the publications are about your specific species, method or location, maybe it is ok with something similar or related?
Could it be that your subject is mostly addressed in non-academic publications? If you are interested in for example inventories, policies and reports, they are not very well covered in the academic databases. In that case you may be better off doing a broad search in for example LUBsearch or even regular Google.
In the beginning of a project, Google Scholar can be very useful. Its advanced search algorithm will give you a quick start and make it easier to find the first few papers that are needed when you start learning about a completely new subject.