Electronic – Optotriac doesn’t turn off

Triacs aren't fit for switching DC. They basically consist of two transistors connected in such a way that they keep each other conducting once the triac is triggered. The only way to switch them off is to cut the current below the hold current level. In AC that's not a problem, since the current will become zero 100 times per second at 50 Hz, but in DC the current keeps flowing.

If it works most of the time it means that in these cases the current will indeed drop below the hold level.

Two ways to switch the triac off: interrupt the current with a normally-closed switch in series. Or have a switch parallel to the triac. Closing that switch will bypass the triac so that it doesn't get current anymore and will switch off. Open the switch again and the motor will switch off.

The network is there to prevent high speed/voltage transients from causing problems, such as spurious triggering of the triac.
Triacs have a dv/dt rating (how quickly the voltage across them changes) above which they may trigger without a gate pulse.

To show how this can cause issues, here is a rough approximation of the circuit without the capacitor:

Here is the simulation, notice when the current approaches zero and the main triac tries to turn off, there is a transient - this spike passes through the control triac, causing a spurious trigger and keeping the main triac turned on.

So we add the capacitor back in:

Now we simulate, and see the circuit functions as expected. The snubber shunts the transient pulse to ground and prevents the main triac from staying on.

Further reading:

2N6073B SPICE Model (used in above LTSpice simulations)

Explanation of maximum ratings for thyristors

Thyristor theory and design considerations