High Voltage MOSFET Controller - Random Failure
Your power device appears to be a PNP transistor, not a MOSFET.  As such, it requires significant base current to become fully saturated.

The datasheet that you linked to says that the minimum Hfe is 20.  You should probably target a forced Beta of 15 to ensure full saturation.

So now you need to tell us what your motor current is.  Then we can help you calculate the required base current.  I'm also guessing that your opto-coupler is way undersized for the base current that you need.

Finally, your schematic diagram is extremely confusing in regards to the polarity markings on the 120 Vdc terminals.  Because this is a PNP device, the Emitter should be the most Positive terminal.

BTW - The transistor symbol in your schematic diagram is the wrong symbol for the device that you are using.  You should be using the symbol that is in the datasheet that you linked to.