Testing a circuit for connection problems can be a difficult and timely task. It requires you to check almost every wire for conductivity of electricity. A lot of times, electronics attract dust and other particles that are harmful to the circuits. This dust can short out the circuits or corrupt the connection. Sometimes the problem area is visible, but other times it is not and you have to check seemingly undamaged connections to find the problem.
Examine the circuit board for any dirty or damaged looking parts. Check the bottom -- or bumpy -- part of the board for any cracks in the bumps. If you seen any cracks you may need to use your soldering iron to fix them.
Remove any dust or dirt from the top of the circuit board to remove any connection blockages. Do this gently to avoid causing further damage.
Use your voltage tester to check any and all connections that you think could cause the problem. If need be, repair them with your soldering iron.