Include fewer keywords, too many keywords can make your search too specific. Try using just the most important concepts. Some protein isoforms have not been heavily researched so you may just need to search the name of the protein in PubMed.
Choose broader keywords. For example, if you can't find anything about a particular protein isoform, consider searching instead for all variants of that protein. Maybe there just isn't much literature on the isoform you are interested in.
Check out the reference lists of good articles that you find - you may want to use those articles too! Reading a good review article is often helpful, because the reference list will likely be quite long and full of other useful sources.
If you look up a relevant article in Google Scholar, you can see how many other articles have cited that article. This can help you find more recent articles on the same topic.
