Let's divide QA(Quality Assurance) from QC(Quality control), check the difference.
Short term solution for manual QC would be to create test plans, acceptance criteria's, force your QC to add test evidence(Screenshots, screencasts, logs, SQL query results), which prove that all ok, you should also plan the number of bugs based on previous development history. For instance, on one of my projects, used only manual QC, there was 0.8-1.2 bug for each 10 hours of development. That means that for 40h feature there must be from 3.2 to 4.8 defects. So I shall plan to find at least to find at least 5 of them.
General advice would be to switch from QC to QA. On all stages of the project. Example.