Can anyone tell me why it's not engaging into forward?

I am a young boat owner. I have an 70's Caravelle with a Johnson 135
Outboard.
Last summer I was on the water and it slipped from forward to neutral
unexpectedly. I shifted it back into forward and after a minute it
slipped again. After two or three times it finally would not engage
into forward fully. I could only go about 5 mph. Reverse was fine and
the motor was running well.

I drained the lower unit oil and found fine metal shavings. I took off
the lower unit. When I shifted it into forward the top drive shaft
would spin, but the lower shaft would not. There was the slipping that
was happening on the water. When I shifted it into reverse the top and
bottom shaft would spin. They were engaging.

I recently broke down the lower unit. The forward gear, the clutch dog,
and the reverse gear were all fine. There were no broken pieces.

Is it the hydromechanical shifter inside the lower unit?

I've heard of some things to check but I don't think they are the
cause:
1) Check the shifter cables and the shifter box. Something may have
come loose.
2) Check the prop. It might be slipping.

Can anyone tell me why it's not engaging into forward?
Thanks, Jason