Clean the trouble optos with a cotton swab and some 90%+ rubbing alcohol.

Barring that, get out a multimeter and put it across the contacts of the trouble optos. See if you can find out if it's the actual optos that aren't firing or if it's the board.

For the actual semiconductor? Very rarely. But vibration in the areas optos are located can take a toll on the connections both internal and external after 15+ years. Optos are relatively cheap and easy to re-manufacture anyway so it seems like its less hassle to replace them most of the time.

For the following tests, set up your multimeter with the machine off. Do the tests with the machine on. Turn it back off between tests as you do setup.

An opto switch is made of two parts. A photo-transistor and an ultra-violet LED. One side is the LED and the other side is the photo-transistor. Make sure the to clean any dirt away that would obstruct the light beam and make sure the LED and photo-transistor are proper aligned first. Proceed with these tests if that doesn't work.

1) Use your multimeter to check if current is flowing through the diode. If yes, go to (2). If no, go to (1b).
1b) If no current flows through the diode, put a 1k resistor in-series with your multimeter. Keep the multimeter in current-reading mode and connect it parallel to the LED. Are you getting current?
Yes) Replace the opto. The LED is bad.
No) The opto board is not supplying current to the LED. Fix or replace it.

2) Check the photo-transistor Emitter-Base voltage. This is done by connecting the leads of your multimeter to the transistor's emitter and base. Break the invisible LED beam a few times with your finger while watching your multimeter. Is the voltage toggling?

3) Set up your multimeter to check the photo-transistor's current on the emitter. Break the invisible LED beam a few times with your finger while watching your multimeter. Is the current toggling?

If (2) or (3) shows no toggling, then there's either a problem with the transistor or the opto board. Most likely the transistor. Further tests hinge on whether or not the photo-transistor is NPN or PNP, which I don't know right now.