Hi there,
You will not always be able to get a higher volume of searches while maintaining the same CTR or costs.
To get more traffic, you should look at using more broad or modified broad match terms, however you must keep an eye on your negative keywords in order to maintain relevance. What keyword types are you currently using?
You should also keep doing keyword research, basically adding in more new keywords as you find them as this should expand your reach. You should also look at fully utilising ad extensions to help draw in that extra traffic.
Ad groups should have their own unique keywords as otherwise they will compete against each other, this also helps you maintain the adverts relevancy.
Let me know if you need further explanation/more help!
Sam