To diablo: Probably, if you runned the circuit with too much voltage (more than 400 V), you burnt the SCR. It was quite usual for me during thqe development tests.

Try changing the SCR.

Another important detail is that if you're using a logic signal Hall device, the output will be reversed (e. g. you have output until the magnet goes to the sensor), so the power stage needs to be adapted to work correctly.

Best regards.