Do the encoders work if you swap them, i.e. does nothing work connected to those encoder 3 & 4 inputs?
Do you have a multimeter with continuity mode? If you do, you can hold it against one of the encoder pins and the common pin. While holding it there, rotate the encoder. You should hear a short beep whenever you click the encoder.
Test both encoder inputs (A and B) this way. Start at the encoder itself, then test again on the bottom of your PCB, where the cables come in. Also check continuity onto the MPC IC itself (from the top, by poking the socket). Use the schematic to check what pins they should be.
That way you can rule out cable/soldering problems.