TRIAC triggering issues?

[definitionofis]

The BT131 needs slightly more current for triggering with reversed polarity on the gate, and with reversed polarity on the switching main terminals.
The specification says 20mA typical and 25mA might be needed for some circumstance.
You are only sending 12mA, you said. 5mA to 10mA works for positive gate polarity, says the specification sheet.

(I came here hunting for gate resistor values for BT131 before I turn my circuit on. I am at 100 ohms and thought I might be too low.)