Where did you get the thyrister from? It could be fake or faulty. Just leave the old one in for now.
Second, did you do that to the board, or was it already in bad condition? We have soldering advice in the beginner's section as bad soldering can also damage components.
It was probably never the triac or thyrister, they usually fail noticably, not with a small voltage drop. Without a schematic, the numbers on the chips would help figure out the circuit.