Another possibility is "wimpy" midi output on your keyboard. For some reason, my M-Audio 49e keyboard does the same thing. try changing the 1200 ohm pullup resistor on the 6N138 output to something like 390 ohms. You can try tack soldering in a parallel resistor on top of the 1200 ohm unit to see if that helps before removing and replacing a resistor. On second thought, if it happens to solve the problem, just leave the parallel resistor.