yes as jonathan has said, putting one output in parallel is not a good idea.
if it has its own output from the parallel port than it is just down to tollerance.
the driver that has the problems is just slighly out of spec or the others just have a higher tollerance.
just power the opto up with a good +5V and problem solved.
Or get a breakout board that buffers the outputs.
Simples...