Understanding various source codes?

I have a huge problem when it comes to programming. I clearly understand and can implement graph, heaps, trees, searching, sorting, string-matching, some image-processing, scheduling, ect algorithms in most situations. I also understand the C, C++, Java, and embly langauge. I have good working knowledge of OS, computer architecture, organization and circuit design. I know numerous of software design patterns. I make excellent grades and I have completed a lot of projects of my own. However, when it comes to reading someone else source code, I blackout. For example: I was looking at a small open source project and I could not understand the code at all. I could not understand the practical use of a many functions, structures, and cles. People tell me to overcome this by reading more source code. I attempted this for over 3 months without any luck. Are there any techniques, methods, or theories to read source code and understand the practical use of a particular segment?